Yes I agree - only the same as the money we've just spent renovating the small strip of front "garden" that we have (photos to come on the house projects fred as just about finished).
One of the very helpful things my questions flushed out was that the previous owner had had a valuation done and the valuer had come up with £1,500!! That allowed me to say I was offering many times the true value...
Technically these sorts of valuations are determined on marriage value and given property in London is so expensive and butting up against affordability constraints, whether we have that additional bit of land or not is not really going to move the dial on price - only really if you have an extra bedroom or significantly larger garden (ours is still just patio of a terraced house, just larger with this piece).