Thanks, everyone. What I'm looking for is something that folds down to small (or at least narrow), so that when not in use it takes up as little space as possible. It's not so much that I want a 4 seater that converts to a 6 seater, but a 2 seater that will sit 6 when extended. It will probably go against a wall when not in use (and maybe when Mr Dog and I are using it), but I don't want it to stick out too far, as there is not a huge amount of space, and there is other furniture in there.
I got rid of a large table that extended to huge as it took up too much space, and replaced it with an old drop-leaf one of my MIL's. It is oval though, so even when up there is a lot of wasted space, and although you can get 6 around it at a push, there is just enough room for plates and glasses, but no space to put out food. It's not really working for us, and the height is wrong for the Ercol chairs that we're using with it. When I decorate it's going to be very much a case of 'out with the old and in with the new', as I am replacing virtually everything in the room.
Re budget - I have an overall budget for the room, rather than for each item, so can rob Peter to pay Paul if necessary. I don't want to blow the whole budget on the table, but if I see the perfect one I can cut back on the cost of something else, and I'd rather have decent quality.