Thanks for the comments/ideas.

I thought that too about the engine and sadly don't have a socket big enough to fit the crank pulley bolt and it's an auto so can't put in gear to move it that way. I can move the pulley a very small amount by hand holding onto the pulley itself, which suggest the engine is OK. As mentioned it was running a couple of months ago no probs and the engine oil is clear and relatively new.
I might try the bypass idea you mentioned to take the starter relay out of the mix but the fact that it's clicking suggests the relay is doing what it should.
I might take the one of the batteries out of my truck as well and try that on there, I know they are good batteries on that