The DVLA isnt as perfect as we might think.

I used own a 53-plate MX5 that, according to the V5, had 3 seats.....!

That sounds like software - "translating" the MS in "Ms Caran Garrod"

Was it letter writtten by an official, perhaps someone in drivers medical branch, to Ms Garrod personally, rather than a standard mailing?

If so autocomplete in Word can do exactly what happened here. In a previous job I had to mention the organisation's founding legislation, the Tribunals, Courts and Enforcement Act 2007, in all sorts of dicuments and correspondence. I created an autocomplete where typing TCE07 would insert full text.

A colleage responsible for minuting meetings did same with a dozen or so officials names. One pair of initials were same as final letters of our postcode - you can guess the rest.

I can well imagine somebody required to use names of medical conditions doing same thing. It can, IIRC, be case sensitive in which case Ms would work in normal way unless miss keyed as MS - or was MS in a database if merging from say an excel spreadsheet.