:clap: YES :clap: YES :clap:
I altered the web forwarding on the domain names from framed to un-framed, which had the effect of show the true url on the browser and PROBLEM SOLVED.
It was as stated above, the url shown was not as PayPal expected and their security system prevented the transaction from going further. It was only happening to a few people as most were, by chance, going through using the path that PayPal recognised and not one of the "covered" routes.
Hope this helps other with a similar problem
Thanks to DR Byte - you're good !
John