I remember there being some members on here with cats and would appreciate some advice.

In January we rescued a kitten from being used as dog bait, when I heard about what was happening I just had to go and help her. We bought her everything she needed the next day. She was very nervous but with a lot of time my son especially has brought her round and she now trusts us and loves her cuddles. She goes out through the day but will only settle in my sons room to sleep. She eats her food there too as that is where she feels comfortable away from our dogs. She has two litter trays in there which are kept clean.

Here is my problem : she keeps weeing on the bed every few days so much so that his mattress is now ruined. I have just bought myself a new bed and the first opportunity she wee'd on that too. I keep my bedding very clean it is washed at least once a week and since she has done this I have washed with biological powder to get rid of any smell. I have also used bicarb and citronella oil solution on the bed which is supposed to deter cats.
This last week once the bed has been made Ive covered it with tin foil as Ive heard this should stop a cat jumping on the bed , however ours loves it and curls up on it to sleep.
I'm at my wits end now wondering how to stop this behaviour, any tips and advice would be so very much appreciated.

Take her to the vet for a check with a wee sample if possible.

Cats have a very concentrated urine and are prone to developing crystals and UTI's especially if they prefer dry food. Inappropriate urination is the first sign there is something wrong as they associated pain with the litter box.

Forgot to add in my first post that I had her tested for a UTI back in April and all was clear. She does use her litter trays but also wees on the beds.

Once cats starts doing something like this it is a very hard habit to break. They seem to love to wee on soft surfaces. So they first thing I'd do is keep all bedroom doors closed to prevent her from getting in there, to break the habit. The second thing I'd wonder about was whether she was stressed by other cats in the area -very common, and one typical reaction is then to pee indoors to show it is her home. So I'd make her an indoor only cat and see if it helped. Has she been spayed? Queens in call will wee everywhere. And it would be worth checking a urine sample again as an UTI can appear out of the blue very quickly, and is commonly brought on by stress. (And yes I too would say avoid dry food, feed wet only, for cats with any type of urinary problems.)

My Siamese did this many years ago. She never did it while I was in the bed thankfully :-)  What I did was keep her out of the bedroom except at night and if I got up to toilet popped her in a cat basket or took her with me.

Thank you for the advice,
She was spayed at 6 months and used to be a house cat, we thought she might be stressed by being kept in so let her out for a while during the day.
I don't think she is stressed by other cats as my mother has an old cat ( she lives next to me  ) and that is always in our garden and they curl up together in the garden. I will try the wet food only as I feed dry in the morning and wet at night.

Another thing to consider if you have the room is to move bedrooms or move the furniture around if a room swap can't be done .
I found with my cats that it does stop the habit as we had one that used to pee behind the TV

I´d ban her from the bedrooms but if you can´t do this then cover the beds with something waterproof so she can´t damage them.  As Goldmali says, some cats do like to pee on soft things (like beds) but I wonder if this could be in part because it is taking them back to kitten-hood and being with mum, otherwise most likely either stress or territorial.

I read all the other answers and I'll add:

April was 5 months ago, I'd Vet check her urine again; her whole self really.  Have fun getting the sample, no advice or ideas for that.  Was that when she was spayed?  It's not as common for female cats to have urinary incontinence after a spay but it can happen.

Are the litter boxes side by side?  Try separating them.  Some cats poop in one, pee in the other and like to keep the two apart.  Is it new bags of litter?  Just as with food, there may have been a change made in the production of the litter.  The company may have added or removed an ingredient and that doesn't sit well with her.

My oldest is nearly 18.  She has arthritis and she has stress from a new cat and 2 kittens we rescued.  She has to be shut in our bedroom each afternoon so I can let the other three out of the basement.  Everyday I put a big tarp over the bed because she did pee on it.  Whether her peeing there stemmed from the physical difficulty of getting down from the bed and lifting her legs up to scale the side of the litter pan or if it's some kind of protest I don't know but I can't be washing bedding every day.  The tarp has worked.  You might try this to save your bedding till you can work out why it's happening for your wee thing and solve it.

Good luck.  Good for you and your son.  Poor wee kitten.

ETA:  I put a big, thick towel over the tarp for Bitty to lie on.  For some reason she has never pee'd on it.

Yes she was checked when I took her to be spayed. I haven't had her retested as it has been constant since we took her in, so was thinking it was maybe behaviour since the last test was clear. I will get her tested again just to make sure though.
Do you know her litter boxes are next to each other, thanks for that tip, I will move them now.
I think I have a tarp in the garage so will go and dig that out now :)

Just thought of another so obvious I hope you are not insulted but..... how often do you clean the litter boxes?  We clean at least once a day, sometimes twice.  Mind you we have 5 cats now (thank you whoever let that unspayed cat out loose to go wild and have her kittens in our back yard.  GRRRrrrr.) and they all use each other's boxes.  One box will get really full, another will be practically empty.   We don't wash out the actual box very often but that might help turn over a new leaf.  I know some people wash them once a week though.

as for getting samples you can buy a little kit from vet very cheap with litter that's non porous but looks like litter . You let her wee as normal and then tip the liquid into the test tube provide and bobs you uncle. I was amazed when I had to do it how easy it was 

I'm not offended at all, I appreciate anyones ideas where I may be going wrong :)

I clean the boxes twice each day and they are washed thoroughly with new litter about every four days. She uses them both but wees and awful lot. I don't know how much cats wee but I think she wees a lot.

Have you spoken with your vet about the number of times/amount she wees?  May be a problem there?

Cats should not wee a lot, they evolved as desert creatures that needed to conserve fluids hence the concentrated urine. My cat with chronic renal failure wees a lot and also drinks a lot, get your vet to check the kidneys.

I will mention to vet when I go next week, she does drink a lot too x

The vet will be able to tell from a physical exam, my girl has been diagnosed 3 years now and does very well on a low protein wet food. x