... they just said that they had to build to the measurements on their plan and this small closure was common
Yes they should build to measurements but for this brick bond (stretcher bond) there is
never ever a cut brick of less than 100mm (half a brick) in the wall and closures are not common at all for this bond.
Basically any cut bricks are either a 1/2 or a 3/4 and are only placed at the wall end or in the centre of a run or under the centre of a window. In this case, what should have been done, is a 3/4 is used on one course up the door jamb, and a header plus 3/4 on the next course to maintain the bond
Also, the air bricks could have been more appropriately spaced - evenly along the length of the wall or at least centrally under the door. Don't be fobbed off with "it has to line up with the one on the house"
And the DPC has not been laid correctly (20mm joint) and is not identifiable - which is another quality issue
The standard of the brickwork is poor, and it is a typical amateur job by someone who does not understand basic brickwork principles. This was not done by a tradesman
Advise the company that it "diminishes the value of the conservatory and your enjoyment of it". It is also contrary to BS 5628: Part 3 - Code of practice for the use of masonry. Also guidance from the Brick Development Association, NHBC Standards (6.1) and any other text on the subject
Demand a reduction in the cost or insist that the brickwork is changed - and the new work properly blended in with any existing brickwork.
If you get no joy, offer to employ an independent expert to comment on the standard of work and if the work is found to be contrary to accepted standards, then the company pays for the survey and for the remedial work or discount the price to reflect the quality of the work done.