In short the PayPal standard feature by itself is a simplistic one so the more complexity you need out of it or that may exist in your current site the more complex the code put into the button needs to be. For example the buttons are based on basic html form code so you can design you buttons to function the way you want as long as the variables/values are valid and are posted to PayPal normally and posted to a top page outside of a frame. There are many ways of doing this but the most universal way is opening the page in its window or tab. The way browsers react to this is specific to browser settings, version, type, and the encoding used on the website it is being directed back to.
Many thank for your response "eelixduppy"
I have corrected the "gap" - (I thought all spaces were ignored).
The following is a direct copy of the PayPal Tech. Reply:-
One example of the is that through my tests of the site through IE8, IE7, and FireFox 3.5 redirect back to the correct page as is without any changes to the code suggesting that the error you are seeing is specific to your browser settings or browser version or possibly add-on associated to the browser you are testing through.
As explained previouse, have now tried on 3 seperate machines, all on IE8, with the same result.
I am sure there must be something I have missed. I cannot conceive that PayPal have never updated their system from IE6 and thereby miss out on the thousands world wide who have IE7 and above!