Unless I am mistaken the transmission for the 1275 has the classic drain on the bottom - fill on the side LBC arrangement. I see you are in Dorset, so can I assume your car is righthand drive?
On the passenger side of the car, on the midline of the transmission bell housing you should see a plug. To fill open the plug and fill to the bottom of the opening. There was good thread on this back arounf 4/10/05 on this board.
I might suggest draining the tranny first (that's the drain plug at the bottom) so that you know a little more about the condition of the transmission, and topping it off with all new fluid.
Good luck.