My Siamese neutered male, has decied to start pulling his hair out in clumps, leaves it over the house and outside, have checked his skin/coat, theres nothing on it, he was wormed recently, and frontlined, as he is every 3mths.
We have another male neutered cat, who have lived together for 5yrs, there are 2 male nautered cats over the lane.
Do you think it could be hormonal or stress of some sort??

I worried hes going to carry on doing it and leave himself bald!

Have you checked his anal glands? Tends to be the most common reason.

How old is your cat?  It may stress but could be thyroid related too.

Naz is about 12yrs old now, he has just started the last couple of days pulling his hair out!
Hes only pulling hair from the middle of his back, no where else!

not being funny but are you utterly certain no fleas?  that makes cats pull fur although saying that siamese are prone to it as a behavioural thing anyway. 
my current one went through phase of doing it to his tail as a youngster. we got him checked out at vet, nothing wrong and no fleas and sprayed the tail with bitter apple which broke the habbit.  can sometimes be through bit of stress or bordem, indoor cats more inclined but it can be breed trait.  same as some get obessive about sucking on wool/cloth- a bad habbit that needs breaking quickly.

There's a LOT of bugs and things about at the moment, virtually all my cats and dogs itch after having been out into the garden, yet there are no obvious signs on the skin.

I have sprayed Naz again with frontline and checked his coat, which is very clean, he has access to outside all the time, his skin does feel hot, can cats get hot spots??

If you want to take the heat out of his skin pat on cornflour, it is good.

Are you sure he hasn't developed an allergy?  One of my papillons developed an allergy to fleas.  Even though he was always kept clean and up to date with Frontline he still suffered - until my vet put him onto some anti-allergy pills.  As long as the medication is kept up there's no problem whatsoever.  With a previous dog (not had this problem with a cat) I found that a veterinary aloe vera lotion/spray worked a treat.  (Forever Living, although loads of other brands available now).