coughed up some electrons that declared:

Toolstation have a decent selection of stuff. One option is the
universal adaptors which replace a nut on their coupler:

http://www.toolstation.com/shop/Plum.../sd2729/p77932

As I remember they replace a nut a size bigger than you'd expect, so
the 15mm goes on a 25mm fitting, the 22mm needs a 32mm.

OK. Thanks for that. I'm veering either towards teh BES fitting,

or this:

They also do MDPE to BSP fittings which would be simpler as long as
you're happy burying a compression to bsp fitting.

Subject to Andrew's warning about dezincification of brass, I also like this
idea.

I will be putting a tiny inspection doobrey over this joint anyway as it's
right next to the house and if it fails, I want to know about it (rather
than waiting for the house to fall over).

Cheers

Tim