Thanks everyone for input.
The chain I have been considering is 6mm x 42mm, £25.00 for 10 metres.
But I think I have worked something out totally incorrect. If we say the circumference chain on each wheel is 27 inches in diameter, it's circumference length is 84.78 inches, 8 of these, 2 for each wheel, that's 678.24 inches of chain, or about 56 foot! Then the tread pieces, about 10 inches in length, 8 per wheel, by 4 wheels, another 320 foot of chain! So 1000 foot of chain!
Have I worked that out wrong???????????