I'm putting a soot door into an external chimney so it can be swept easily once the stove is in. Once I've cut the hole (angle grinder) what's going to be the best thing to hold this thing in? Gripfill, mortar?

The door is 9" square, about 3" deep and has a cast frame if that makes a difference.

You might be better off posting here:

http://www.diynot.com/forums/

Pretty knowledgeable bunch

Epoxy Resin?

no nails

Screws and raw plugs?

My mum managed that a few years back.

She drove my Volkswagen Polo into the garage wall.

I take it you have purchased said hatch? Did it come with installation instructions if so follow those. Failing that use good old fashioned mortar. Make sure you get the mix right to match the existing brickwork, nothing worse than when the mix is too strong and it stands out like racing dogs bollocks. Most likely 5/1 (i.e 5 parts building sand to 1 part cement).

Dont mix it too wet otherwise you end up with it smeared allover the surrounding brickwork, rule of thumb is that the mortar should spread like butter when you run your trowel through it.

If not give Rusty Trowel on here a page as he is bang up to date on how it should be done!

MUTDs (Ex-Brickie)