I have a couple of pups at 8 weeks which appear to have developed a urine problem. One actually went to its new home and reported this and now one I have until the 19th is the same.
Squatting but not doing anything. My pup appears OK. I have got them to the vet and have antibiotics but wondered whether anyone has had experience of this as not something I have had before,

This is different to pup that was earlier. Why would they develop this?

Well having a urinary tract problem at 8 weeks isn't 'unknown'.   A little bitch I bought in, my first buy in after years of breeding my own and nursing the final two of my bloodline through their cancers for over a year, so the last thing I needed was more heartbreak, came to us at 8 weeks.   Yes puppies urinate lots, but this one was peeing for England, and not always producing much.  Soooo, having collected some urine, it was off to the vet who found her urine was 'all wrong'.   She was put on antibiotics but over that weekend there was absolutely no improvement.  My washing machine was on the go all the time and she had to be crated, which she hated, more than I'd have wanted, or sit with me to keep her up off the carpet.    When I took her back my then vet started talking about the possibility of actual stones (there were crystals showing up) at which point, again given this was supposed to be a new start, I phoned her breeder and said I was bringing her back.  To be honest, I'd intended for her to get what was going on sorted out, and let me have her back.  When we got there, she slapped a cheque for the full amount at me, and that was it.  I later heard 'the others' - those she still had, were 'the same'.   What that meant I don't know but going by the KCSB records, that little bitch went to two more homes (transfers showed) before she presumably 'stuck'. 

Could be immature bladder but the fact they are squatting but producing nothing, would have me concerned, for sure.  Sorry I can't help you, but yes, UTI that young is possible.

I would guess in a litter a bacteria is easily shared. I've had a puppy with uti at around 7 weeks and she was doing what you describe. Few days antibiotics sorted it and no further issues, she is now 4.5 years old.