I know it fits but is a 10A plug in a 15A socket legal?
Absolutely, they are designed to do exactly that. A 10A plug will go into a 10A, 15A, 20A or 32A socket outlet. A 15A will go into a 15A, 20A or 32A one, and a 20A will go into a 20A or 32A one, and funnily enough a 32A one will only go into a 32A socket outlet. Pretty much foolproof (unless of course people make their own leads, but I won't go into that)
Go into any caravan park and see how many 15 Amp extension leads are in use, I would suggest it's a very low %
Question: how many guys here use a 15 Amp extension lead in caravan parks
PLENTY. I don't go into caravan parks that often, but just about every caravan I've seen uses a 15A lead.
And I have a 15A outlet (4 in fact) in my house, so using a 15A lead is no problem for me. My camper has hard wired 240V systems, and is fitted with a 15A inlet, so I use a 15A lead.