Solved it.
My own fault.
To protect the workshop I always set the grill on a large glass chopping board. The back feet were off the chopping board and so altered the slant of the base, when the fat dripped off some of it was rolling onto the underside beyond the drip tray.
annsixty - thanks for your offer.
chelseababy - The removable plates are brilliant but I don't think it's as hot as the old type and is a little slower. However it still is excellent and I love it and use it all the time.
rosesarered - thank you for your suggestion, I'm sorted. I did see that the drip tray can be bought on line for about £2.50.