@ste
Sorry, forgot to get back to you,
Assuming your recess is brick, if it's wood different measurements apply.
So, if your fire is in a recess it needs 150mm to the sides,
100mm (but can be less sometimes) to the rear,
and 600mm above, depending on lintel material this can be less.
To the front, 300mm is the absolute minimum, but it should extend to cover the door opening if the door is larger than 300mm.
He's not 100% on the 250mm thing, that is if it's on a wooden floor, but there are other accepted methods of doing it without it being 250mm deep.
I'll wait for more info off him this evening hopefully, he was prob watching the football yesterday.