Hi when fitting a new smaller width fire door into an existing opening 10mm max each side what is the best method to fill a gap where the old door frame went into the plaster sorry for the poor drawing but if red is the wall with a 10mm dip in it where the existing frame went in and blue is the new door frame would a plywood strip (yellow) be ok to fit to the wall (10mm less is width either side of the frame) and then mastic sealed flush to frame

Assuming the door leaf is timber-based and the wall is solid blockwork/masonry etc.

If you work to the British Standard BS 8214: 2026 and document the installation work as to that Standard, you can evidence the compliance of the installation. So consult that document, page 32 for walls of solid blockwork/masonry.

So from what you have said, the void left by removal of the old door frame is 10mm max depth by whatever the width of the frame was. Therefore you may install the new door frame to entirely cover the void with that void tightly packed with stone wool and then intumescent mastic at each end as detailed in BS 8214 above.

Do not leave any gaps or voids at all. Make sure the new door frame extends over the void so that it's completely enclosed, is of the correct material and is above the minimum sectional dimensions as stated in the installation data sheet for the fire door leaf and is fixed with suitable fixing screws at the correct penetration and fixing centres. Again, all as detailed in the installation data sheet for the particular fire door leaf you are installing.

All of the remaining installation work, including all hardware, seals, ironmongery, gap tolerances and trim allowances must meet the requirements of the installation data sheet for the particular fire door leaf you are installing.

So that you can be compliant, if in doubt consult a fire door specialist.