Can you advise the make and model of your oven?
Many thanks, ***** ***** the glass lamp housing is intact (not broken or cracked), and you have tried using your hand to rotate the housing anti clockwise, and the tool from the service centre but in both cases it would move?
The likely cause is a build up of grease, firstly I would get some penetrating fluid such as WD40 and spray around the housing where it makes contact with the back of the oven (I would do this 3 times over the course of an hour). Using a towel, over the cover I would then try and use sharp movements anti clockwise to see if you can free it.
If this doesnt work, I would then try some grease such as Over Pride, and do the same, and if this doesnt work its a case of stripping the unit down and removing the bulb from the back, however would suggest you get a engineer to do this, as if your note faceful you can damage the lamp socket.
Please let me know how you get on.
I havent heard back from you in the last few days, and just wanted to check if you still require assistance, I would appreciate it if you could rate the service received.