Well after almost a month I've finally had some success. We'd had normal mouse traps down with Mars bar as the bait, and not a sniff. The pros came and put 'proper' poison down in every room and after all this time the mice haven't even touched it - though the poison 'bags' are full of rodent hair where they've been running over it.
We've seen them in the living room, kitchen, our
bedroom 
and pretty much every room in between. The terriers wake us up nightly, usually around 2am, yapping at mice running around their indoor 'box' (crate). Every time the dogs come into the kitchen (while I'm getting their food read), they 'mark' (indicate rodents) very strongly in one particular corner of the skirting under the kitchen cupboards. I can't take the boarding out to get traps/poison down because it's a rental property, and the landlord has been next to useless. I reported this issue a month ago and they've not even bothered to return my requests for help with rodent proofing the actual property.
Last night I finally snapped. I found droppings around our bed. Urgh. I've spent £15 on more B&Q mouse traps, £6 on Eradibait, £5 on 50 little bait stations to put it in, and £3 on 4x glue traps. I changed my bait from Mars bar to a broken piece of Hob Nob dipped in peanut butter, and not an hour later:
That's not the one I've seen (which is black), nor the one my Mrs has seen (which is bigger, and darker brown). As some may recognise, that's a common House Mouse - not a Field Mouse as the pest controller suspected we had. Still, one down, eleventy million to go. :\
EDIT: Oh, and for the interested... that particular trap was located not 3" from the end of our bed. Ick.
