[rab] How exactly does that system work from the seller's point of view, then? Is there something that could stop you from dropping your offer down to £10, for instance?

Yes, they could simply refuse.

...and then the seller has to weigh up the relative pros and cons of accepting a lower offer than the original one, or putting the place back on the market. In our case, I don't anticipate any problems with the survey - but it's nice to know that we can pull out if the surveyor says the place is about to fall down, but they can't suddenly decide they're not selling to us after all.

Actually, I now understand we haven't quite reached that stage yet. The seller now has to agree in writing, so until we get that it could all be orrf.

Just a (necessarily) brief note to inform that I have broken a couple of bones in my left hand, so I might find myself not posting very much for a few days. I'm always surprised at how typing one-handed reduces one's speed by so much more than 50%, and how sore a broken bone can be.