It's really now a matter of finishing off some jobs before commencing the mineral wool filling and drylining of the room. The jobs for this weekend were to make the hole for the fresh air inlet and to remove the existing outer door frame and build its replacement.
For the fresh air inlet I bought an air-brick which I can simply put in the place of an existing brick. This was fairly easy to do drilling round the brick with an 8mm masonry bit removing the mortar. I could certainly tell where I had re-pointed the brickwork as it was tougher to drill than the old mortar - which is like powder. I thought I'd better add: the old mortar is now mostly on the external side of the brickwork as, so far, I've re-pointed only on the inside. I will eventually re-point the outside as well. Once all cleaned out I mixed up sufficient mortar and set the air-brick in place packing it well around. The air-brick was only about a 3/4 brick deep so from the inside I packed more mortar and created chamfered leading edges to the internal surface which created quite a neat finish and will help achieve a good seal between my duct.
Next I set about replacing the existing outer door frame. This was reasonably easily achieved by making some horizontal cuts into the frame and with the help of the claw hammer I was able to pry it away and once loose it came away with little effort leaving the raw brick doorway exposed. I then removed any loose material and cut some protruding nails flush using the angle grinder leaving a clean surface. Upon measuring the gap I found that there was a good 3-4cm difference so I would have to make the new frame to the lower (88.5cm) and pack out the surrounding gaps.
I chose to make the frame in the same way that I had made the internal one which seemed to produce a good square solid frame. This would mean a complete 4-sided frame instead of the 3-sided of the original, this allows the door to jam all round. This was a much quicker process than the first frame obviously - maybe I should go into production...anybody need a door frame? All went well and with a few tweaks in height the frame fitted into the gap quite well...well considering the inconsistent shape of the brickwork anyway!! This was jigged glued and screwed and left overnight. I then roughly secured the frame in situ and held in place with a few screws. I need to decide on the best method of securing the frame in place. I have a few ideas but for now I'm happy with the result. Anyway I think the photos will tell a better story: