I know Kingspan do insulated PB, but do they do insulated PB with 50mm
of insulation and a vapour barrier, which can be dot & dabbed?

The Kingspan website is just fine, with a slightly dodgy menu.

The item you are looking for is Kooltherm K17.

http://www.insulation.kingspan.com/newdiv/k17.htm

The 62.5mm board has all you need. 50mm of insulation, vapour barrier,
12.5mm plasterboard and dot & dabbable.

Christian.