In reply to Tigger:
This came up on a few houses that I looked and I'd never heard of it before. It's very common in and around Manchester, where it's sometimes referred to as Chief Rent.
According to the solicitors, it was brought about during the industrial revolution, when the property building market was booming and developers didn't have the case to buy all the land required. As a confab, it was written into the deeds that for the rest of time, a rentcharge would be paid by the property owner to the former landowner (or who ever has bought out the right to collect the rentcharge), though they failed to write in inflation clauses, hence the tiny amounts pauyable (<£1 in some cases).
A conveyancing solicior should be able to help find who the rentcharge is payable to. If they can't, it's worth taking out indemnity insurance (should be <£50) to cover you in the event that the rentcharge creditor comes out of the woodwork in future and tries to take you to court for loss or earnings.
You also have the right to permanently buy out or "redeem" the rentcharge, making the issue go away permanently.