Many thanks for all your comments and advice.
After reading your comments, I think I have now isolated the problem. Following a number of tests I found that I was not getting a proper second press of the shutter - it was down to my remote sticking or not operating properly.
I have changed the remote and all seems to be working fine.
On the question of CF Cards, I have lost the odd first shot on my 80x 2Gb ones and I now have an RMA number to return them to Lexar. No problems with older ones.
Again, many thanks for all your responses and help.
Neil