watford is on the silverlink metro service running out of euston (on the birmingham line!). however you can get a quick train from willesden junction to watford, so you could get a train stratford to gospel oak (right?) then change to silverlink to willesden, then another silverlink to watfrod. that is, if you like silverlink loads! other wise, the best thing is to get to euston, unless you are north west london. the trouble with the in-london bits of silverlink (ie richmond-woolwich, stratford etc) is they have a shit service and end early at night. so i reckon whereever you are in london, get to euston, get the train up (trains northbound run till about 1.30), then get one back fomr 4.10 onwards, every 20 mins or so. only takes 25 mins to get back to euston. for some reason the last train in the night is a 0.41 back to euston, instead of a 2.00. that sucks, but make it an allnighter!
there is a tube in watford as well, but dont know what time thats starts/ends. takes about an hour i think.