The "underground" stuff may be 20 or 25 mm, if so you should be able to get
fittings to copper at farm supplies shop. They are bulky but the really good
thing about them is that they are designed to be connected up without
isolation, unlike "domestic" stuff. They have O ring seals, with plastic to
plastic you seem to get away without putting in the "insert" to support the
pipe.

Also available from www.bes.ltd