Hello Nick,
I was in much the same situation as you - the surveyors put my renovated 1980’s house in category A air tightness and calculated a heat loss of 9.64kW which made their recommended HP above an 8kW Daikin to an 11kW - which is widely accepted here to be a less desirable HP in terms of efficiency.
I pushed back on the installer saying I thought the air tightness should be better than CatA but they wouldn’t budge, quoting “MCS regulations / obligations” etc - same as yours.
I had a blower-door test done which reported air tightness better than Category C which brought the heat loss to 6.99kW which is within 8kW HP territory.
Still no movement from the installer.
I then phoned MCS asking if they did in fact insist on blind adherance to building date when categorising air-tightness in heat loss calcs. The rep was very open and helpful, saying they had no such proscriptive rules. She went away and dug out their paperwork and emailed me a relevant extract, saying “We base our ACH’s on CIBSE Domestic Heating Design Guide”
…and highlighted the note:
“For buildings where extra attention has been given to reducing ventilation losses…it is recommended to seek specialist advice”
In this context, I presented the MCS guidance and the blower-door test as specialist advice to the installer and at this point they had a change of heart and became able to fit the 8kW HP, which is working well for us 
Everyone has signed off on the installation and the BUS grant has been paid.
So my advice to you is : get a blower door test done and then you can be sure you are not mistaken when you try to convice the installer that you calcs are valid and that MCS will accept them.
Good luck, David.