I have managed to resolve this issue :)
I had spoken to PayPal who could not give me a clear indication as to the problem but upon rechecking the FULL REQUEST I noticed that the expiry year for the credit card was a four digit number so I tried to put a transaction through with just a two digit number (11 for 2011) and this worked.
So it appears that PayPal on my Website payments pro UK account will not accept four digits for the year expiry. I changed the input form element from a text field to a list menu listing years from 2010-2016 and setting their values to two digits. This has worked perfectly.
Many thanks for your help and I hope this helps others with a similar problem.