I wonder if anyone has any ideas about this.

Isla is six and a half years old and was spayed when she was about 18 months. During the last year she has had 3 urine infections, all cleared up quickly as soon as she had antibiotics. Prior to this she had never had any urine problems. My vet decided he would like to investigate further and today she went in to have an x ray and ultrasound of her bladder.
Bladder looked fine and no stones were seen. However he did notice that the neck of the bladder was wider then normal and set further back, he doesn’t know why. A further catheterised wee sample has been sent off and I’m seeing him again a week on Friday. I queried spay incontinance but that was not thought to be causing the repeated infections.

Any ideas?

You won't like hearing about my experience with one of my girls who had this, UTIs ongoing.  This kept happening and apparently cleared, only for this to recur.   Scan was done and nothing seen.  I asked about x-ray and was told 'scans are good enough'.   Hum.  I phoned my vet back upcountry and she suggested that given what I told her, she'd def. x-ray and probably open her up to find a proper diagnosis.   I also took her to see the vet my sister used and his response was as with my up country vet.    So back to the practice.   In the meantime, I noticed her limbs were thickening a lot (even for a well-boned Basset).   This was eventually said to be Marie's disease. 

Anyhow, the vet there I'd been seeing said 'there was nothing more he could do other than refer us to a specialist up in Bristol'.
 
By that time, when we went back to the treating practice, the Senior Partner appeared, did the X-RAYS, where it was found she had numerous tumours in her chest cavity.   We had to give her peace.  Clearly the tumours were secondaries and the prime was probably somewhere in the bladder area.

All this was going on for pretty much an entire year.  She was the last bitch of our bloodline and the second last of all of them - her brother was as heartbroken as we were that this was the final result.   He lived for another year before oral cancer claimed him too.

This was my experience with repeated UTIs and may have nothing whatsoever to do with what's going on with your dog.  I'd go for a second opinion for sure.

Jodi, as a first thing to try, put her on Cranimals cranberry powder.  Cranberry powder is a well known preventative for UTIs.  It won't treat them once the dog has them, but if you put her on it before that, it will prevent them pretty effectively.

My oldie kept getting UTIs, till we put her on it... then no more.  We had 3 in a year, then she lived another 2 years without any UTIs on the cranberry powder...

I am giving her a daily dose of cranberry, when I think she is showing signs of a uti then I dose up with b mannose. Eventually it reaches a point where a vet visit is needed

Hypothyroidism can be a factor too, so this would be worth checking if it hasn't been looked at already.  And it doesn't need to be an obvious case in the numbers.  My lab had repeated UTIs, then was tested for everything via bloods and nothing found.  I asked to see the results and noticed that her free T4 was at the low end of normal.  Vet said it was fine, but I've had too much experience of it.  I ran it by a thyroid expert, who told me about the possible UTI link and confirmed that the result was low for Saffi (based on breed, age, etc), and I insisted on a trial of treatment.  No more problems.

Are there any other symptoms with hypothyroidism I should be aware of?

Take a look at Jean Dodd's book:  https://www.amazon.co.uk/Canine-Thyroid-Epidemic-Answers-Need/dp/1617810169

^ That!

But, it varies.  You don't always see classical symptoms alongside the unusual ones.  Saffi was overweight, but she was also a labrador from a show line who were all obese, so that could have been genetics.  I've also had a rott cross be hypothyroid with no symptoms at all apart from lunging at a dog one day which was completely out of character for her.