I've managed to get it to work today as far as returning to the success page back on my website so yes Rob, it seems that this should be applicable to others upgrading to SagePay 3.0
Only problem I have now is with my success page identifying the VendorTXCode using $Protx_VSPForm_Bindings["VendorTxCode"]
Prior to the update I used this while updating the successful order summary in my website's database to include the VendorTXCode with the order. But now it seems to not be returning any value.
Not sure if I should start a new post regarding this or if Ray or somebody else at WebAssist would like me to post up the details here since it seems to me to be related to the topic