Thanks for the replies, you're right that the part numbers don't match but as Bowie said the two engines are closely related. The V6 is actually called the KV6.
In America I've found a place that sell the pistons and say they will fit both engines...
http://miamibritish.com/shop/land-rover-piston-rings-assy-set-freelander-1-8l-4cyl-lfl000790l-all-makes/