I can't open the oven door when it's working, I suspect, because of the amount of stuff that's stuck to the ceiling inside. It sends a cloud of smoke up which sets the fire alarms off. I think it's to do with a roast I made recently. 
I've never cleaned the whole oven before, just a cursory wipe over the glass
I really don't want to fork out for someone professional to do it although the idea appeals immensely. So what are your top MN tips to do the job quickly & easily?
(I tried the dishwasher cube thing once but it scratched the internal glass so I won't be doing that again.)