anyone know where to get pots of this size - 4-5 gallon, 18 odd litres.
cheers

Contact any local landscapers. They often have all sorts of pots left over
after planting up jobs. Try either talking to someone you see while you're
about, or give some a ring. You may get told to sod off, or that they'll sell
one to you, but you should find someone who'll be happy to get shot of the
things and get them out of the way.

Best of luck.

Dave.