Hi everyone, just wanted to reply with a wee update.
Thank you so much for all your help, advice and suggestions.
Pipsqueak has been at the vet this morning and gone for x-ray and sedation, no obvious sign of bladder stone in the first two x-rays (they did these with her in a box to keep her still) then said they would need to sedate her to get an x-ray of her on her side - no sign of bladder stone showed in the first two x-rays, more likely to be sludge but the x-ray of her on her side may reveal a stone.
The vet said that sludge is more annoying to clear, and that if it was a bladder stone they should be able to, if it is small enough and in the urethra, they can go in and sort of manually remove it. We are just waiting for a phone call now to see what the next step is, sludge or stone.
I will say I was extremely happy with the expertise of our vet this morning, she is so clearly experienced with guinea pigs, she was so sweet with Pipsqueak and very reassuring (stating that she has done bladder stone removal in the urethra loads of times successfully!) and understood my concerns about high calcium foods and everything. I felt so reassured when we left, her name was Laura and she is at MBM Vets in Ayrshire if this is useful to anybody else. She even said she would try her best to get some of the price down for us.