I'm sure it's not legally binding, but equally sure it's a very very good idea to get the conversion training. There's never been a minimum hours, but again, I'd be amazed by anybody going either way within 5 hours. CAP53 which is in LASORS I think is the current definitive document. The crossover is called the ACST, or "Alternative Control Systems Test".
Whilst the ACST is mentioned in there, there isn't any mention at-all of differences training for VP prop, twins or nosewheel.v.tailwheel, all of which exist within the UK microlight fleet.
G