(from another WH resident), haven't got a caravan myself, but heres where I've seen them.
Farm on the Guilsborough Road on the right about 1/4 mile after the new roundabout.
Farm on Station Rd (thats the road to Watford), on the right about 1/4 mile before the railway bridge (one of my neighbours leaves his caravan there).
Bit more secure (fenced etc). Mr Threadgold at Welton Lodge Fm nr Watford. Go over the A5 crossroads towards Welton, first right and the farm entrance is about 100 yds on your left 01327 703646