Had a crack at this last night (starting about 7:15pm) and posted 59.35. Thought I'd gone off too slowly but actually my lap times got progressively slower after the first lap of 19:15.
There was a nagging headwind on the drag up to Richmond, and I got seriously held up by cars on the section between Robin Hood and Roehampton. But them's the breaks - it was all good fun and gives me something to chip away at.