Fill in this form and send it to the relevant Land Registry.
Search facility here...
http://www.landreg.gov.uk/regional/
Unless it brings up more than 20 titles then the search will be free.
http://www.landregistry.gov.uk/assets/library/ documents/sim.pdf
You need to send a plan with it, preferably a scaled official plan with a north point and identifying features such as roads and other buildings. If you have a title plan of your property which shows the land in question this plan would be ideal to copy and mark.
You can identity the land as land adjoining x road, area, town/city. You need to mark the relevant area on the plan, it is commonly edged red and sometimes hatched if this makes it clearer.
The result will tell you whether any of the land is registered and which bits and the title numbers of any relevant parcels of registered land.
With these title numbers you can then request copies of the title register and plans from the Land Registry to see who owns the land.
Hope this helps.