AAAhh so! Another bit of wrong information to send me down a path with no hope. I didn't dare take out another bolt from my car to check the spec and so relied on the forum which had a post in another thread stating it was pitch 2. Most bolts in the automotive trade are 1.5 for drive-line (BMW, JLR, Vauxhall). I will now search for a 1.5 x 50mm.
Thanks for that Andrew, the information is more reassuring.
Kev