Having done a search before posting this I noticed there was an old post regarding the maximum limit of £75 cashback being able to be earnt in any "rolling" 12 month period with a BT credit card. The post was designated "solved" as the person concerned had received an explanation regarding old and new credit cards - new were recived after 2009.
Today having spent over £800 this month on my "new" card I started using in 2010 MBNA informed me that I had exceeded the allowable £75 rebate and couldn't get a penny off my bill this month!!
JUST CHECKED AGAIN and I must have had my card before the 2009 deadline as I seem to have been getting the higher rate rebates - just been getting too many and so fell foul of the £750 limit.
I'M HAPPY NOW as it seems virtually impossible to get more money using the "new" card!