I have found an answer from a local dealer.
Oven top grills are static and don't hinge down.
Humberside Domestic Appliances, Grimsby suggest the following:-
Fill a large baking tin or tray with water and place in oven. Switch on oven to highest setting and leave for 30min.
Switch off and as soon as you are able remove the baking pan and clean grill and roof as best you can. It should act as a steam clean. Avoid burning yourself!
Have not tried this myself yet but aim to do so ASAP.
Hope this helps.
Mrs. S. McIntosh, Scartho, Grimsby.
November 2011