Very simple to solve.

Mot section 7.1
method of inspection 3:
On petrol engine vehicles that qualify for a full catalyst emissions test, check the presence of the catalytic converter. Note: To ascertain whether a vehicle qualifies for a full catalyst test, use the flow charts in Section 7.3.C disregarding the result of the Basic Emissions Test.

Reason for failure part 3:
3. A catalytic converter missing where one was fitted as standard

So quite simply the check to see if cat is present is only relevant if the engine requires the cat test, which a pre 1991 engine wont.

So your mot man is wrong.

You dont need a letter from ford either. Who in ford would you write to for a start! and who in ford would keep such a database readily available for public access??

You get a nice freebee with EVERY burton power catalogue and thats the engine date table. Your MoT man would have to be a right cock to reject a well known respected published data source like that.