Went to Leeds today so now I can report back with reviews!
H&M - not as much maternity stuff as I was expecting/hoping for, none of the jeans fitted me well but I got a shirt and a wrap cardigan for £10 each.
Next - went in to get some Postman Pat pants for DS (attempting potty training at easter) which they didn't have, checked just to make sure and nothing, woman I spoke to confirmed nearest was Birstall (I'm carless atm so not much use!)
New Look - no overbump jeans but some supercheap £12 underbump ones with side panel. There's no way these were going to stay up on me though! Got a nice floaty smock top for £14.
Zara - blink and you miss it maternity rail - I had to ask where it was after searching the store! No jeans.
Gap - Great selection of jeans, over and under bump. If, that is, you are a size 4 . I did manage to find one pair of overbump jeans that I just fit into (US 10/UK 14) so I bought them anyway. Even though I suspect I'll outgrow them totally in a month or so I guess I'll get some use out of them after the birth too! They were £30, so not too expensive.
Dorothy Perkins - no overbump jeans or overbump other trousers either, and their tops were all in really cheap-looking jersey so didn't get anything there.
I suspect I'll eventually end up ordering some from Next Directory, which I was really trying to avoid [sigh] - apart from the sizing issue, I've never joined as I don't like buying on credit (don't have any store cards etc) and I really don't understand why they don't stock the maternity stuff in store, it's really very annoying (KS tries to avoid swearing...)
Thanks for all your help and advice!