I have bought 18, 19, & 20s. I find the 19 the best compromise. I don't have a model shop to hand that stocks these for obvious reasons so I just order 19s now so to be assured of minimal running problems and having to wait until they do another order.
I think I have used the #5 on a Dapol wagon and also on some older Hornby wagons that use the riveted on tension lock. I have also used it on a Bachmann ballast wagon that had the cranked tension locks.
I think I also picked up some whisker 146s. not sure why though, mainly to experiment but I think they work on the Hornby MK2 & 3s.