Nathan, the person that taught me to shoot as a teenager would have known your Dad, as he was a fellow D11 Instructor at the time the Enforcer was introduced. Sadly, 45 years later I can't remember his name, although I can still 'see his face'. He was one of the D11 instructors that deployed with the Enforcer at the Balcombe Street Siege in Dec 1975, so it's possible so did your Grandfather?
My father was a serving Met.Police dog handler at the time, as the D11 instructor that taught me .22 ran a local small bore club to where we lived and had been deployed to Heathrow Airport in the early days of the IRA actively there when they were using MOD supplied No.4T's before the Enforcer was introduced, which is how my father had got to know him.
As Gil suggest above, contact the Met and they should be able to help.