I've just done it this morning as it happens. It took me 25 mins to get there and 25 back. After Crystal Palace follow Church Rd to the lights, Right onto Beulah hill then immediate left down Grange Rd, all the way down onto Whitehorse Rd, right at a big junction onto Windmill Rd all the way down to Sumner Rd, brings you out on Mitcham Rd just at the big roundabout onto Purley Way. Very little traffic and pretty much straight all the way.