Thanks for all the advice. I had a go at it last night, I've more or less chiseled a 1 cm gap around the whole fireplace and I found the two fixings to the wall. I do have a baxi firesystem, so next I have to ring the gaspeople to disconnect the whole thing and then I can take it out. Some plastering will be needed, as we will replace it with a more modern gasfirefront, but I'll do that myself, been plastering over stippled walls for the last while... Not too worried about dust or anything, just bought the house and it's a building site more than anything else!
Again I'll keep you posted.