Earlier posting from Big Wallop:
:So it's just an old water tank for the back of an open fire ? That won't :cause any problems at all, so you'd be able to use a timber moulding fixed :with No More Nails around the outside of the opening. Cheap. Cheerful, and :can be finished in any style you wish.
A DIY job complete! Stinky 70s fireplace ripped out, big hole cement in, got a pro to plaster, built the bottom up to make hole in wall fireplace. Lined the base with 65p slate look vinyl tiles Finished off with chamfered wood cut into a frame. No tools only a hacksaw and a page folded on the diagonal to make 45 degrees, much giggling and 4 wonky bits of frame later, needed a load of polyfilla in the edges. Stuck on with No More Nails as advised. Now all painted up and it looks FAN-TAS-TIC. 2 poofs off million £ eat your heart out!
Thanks for everyone's advice Suzanne