Seems we can do it for £36.45 for two adults and one child (including Network railcard) from Burgess Hill
London travelcard for zones 1-6(Thamelink) and then return from Harrow and Wealdstone to Watford Junction.
Cheaper to get travelcard just for Thameslink as I understand you can use it on...
So, can you buy a travelcard for zones 1-6 and then a return from Harrow and Wealdstone to Watford (even if none of the trains you use stop at Harrow and Wealdstone)?