Smartalec. I contacted Neff customer service and received this reply.
Dear Mr Smith,
Thank you for your email.
Disconnect the appliance from the mains. The lamp cover should be turned anti-clockwise as you look at it from the bottom.
If you are having difficulty removing the cover you could try warming the oven up slightly, then isolate the power and allow to cool enough to be comfortable to handle, then wearing a rubber washing-up glove to aid your grip try to remove the cover again.
Another method to try would be to first remove the oven door (as this will enable you to access the cover more easily), place a tea-towel onto the floor of the oven and then with a small hammer ( yes, we know it sounds drastic) very gently tap the glass cover around the edge. Give it a turn, tap again a little harder, give it another turn. Repeat this process until the cover unscrews. It is quite normal to experience an initial resistance when removing the glass cover for the first time. Once achieved, you will find subsequent removals easier.
Alternatively, use something like the Boa Constrictor strap wrench, usually used for removing stiff jam jar lids and the like.
I havent tried anything yet but I will this weekend ( I'm not so sure if I will try the hammer though ). Hope this helps.
Keith
Keith Smith, December 2008