I'd imagine the land registry would be a good port of call as in theory they list who owns what, but that will depend a lot on whether it's registered yet. Apart from that, possibly the council might have details of land use.
As for claiming it, if you can't find anything about it and depending how well your house deeds are written you might be able to fence it off, then wait a few years and if no one objects you can in theory claim it as yours.
Can't guarantee the absolute legality of this mind you, but we did this along with a number of other houses since our deeds said the land went to the stream, but since the builders moved the stream then it slowly got claimed and is now fenced off. You might be unlucky however and have useful deeds.
http://www.landregisteronline.gov.uk/
Trev