£15 back then in today's money is about £300.
I'm guessing the equivalent car today would be a 20 odd year old Mondeo.
You could possibly insure that for £300 in some parts of the country.
On the inflation thing, it's just number changing, but...
I assume it doesn't matter if you're holding it with the foot brake or the hand brake?
My guess it only occurs at a certain RPM which only happen under stationary idle under load.
In which case it could be any of the many brackets in the engine...