Thanks for all your replies.
I finally managed to find the splines on the freewheel last night. They were being covered by a spacer on the hub which just needed a bit of force to pull it out of the way.
None of my cassette tools would fit it so I took it over to Halford where the chap in the Bike Hut had a go at it with a very long wrench. Unfortunately it was still too much for him and he had to admit defeat. He did say that the gears were still looking in pretty good nick (Oh the shame of taking advice from the bloke in Halfords!) so I'll probably leave it on the wheel for the time being and just change the chain .
Thanks again
Chris