My legs gave up on the hill. Why do up hills feel longer every year? Who were the wonderful wheelchair duo? They were wooshing through the water and still able to give me lots of encouragment. Very humbling.
I'm only an out-of-towner, but it looks to me from the Transport for London website http://www.tfl.gov.uk/tfl/livetravelnews/realtime/tube/track-closures.pdf that the Metropolitan underground Harrow-on-the-Hill to Watford is replaced by a bus service. Still OK, I think, to get to the start by going to Uxbridge, but any underground travel to or from Watford will in fact be by bus.
It will be good to get past the hassle of getting there and start running a nice flat 1/2 marathon!