As above you can do a search at the Land Registry.
It won't be searchable on the online database on the website if it hasn't got a number and postcode, however, you can send in Form SIM (Search of the Index Map) with a plan attached marking the area (must be properly identifiable so as clear and referenced - ie to public highways - as possible, scaled and with a North point - preferably Ordanance Survey) to be searched.
If you have a title plan for your property, this would be idea to use if you can mark the additional land clearly. It will also show them an adjoining title number.
They used to be free but now cost £5 (unless over title numbers are revealed in which case it is an extra £3 per extra up to 10 titles).
Note that it will only show up registered title numbers, if the land (or parts of it) is not registered at the LR then there will be no title number for that part so much more difficult to trace.
They will send a result with the title numbers on and you can then get copies to check the registered owners. I'd recommend asking for a plan back to see which bits are owned under which separate title numbers, especially if your plan isn't exact as if the boundaries run a little wide they can sometimes include slivers of adjoining property, bumping up the title number count (and potential price, especially when getting copies of title registers) and confusing the matter.
Form:
http://www.landreg.go...ary/documents/sim.pdf
You can describe it as "land adjoining/to the rear of (house numbers), (street)" etc... and refer it to section 6 which refers to the plan (eg edged red/hatched black).
Hope this makes sense. Let me know if you need anything explaining more fully.