Make it clear to them they have two choices.  They can fix your car, or
they can give you your money back.

That is there obligation.  be nice about.  If they don't come across,
then unfortunately seeking legal advice is probably your next move...

Marty

This shop is dragging this out in the hope that you get discouraged (and
their problem) fades away. Their warranty is no longer 12 months, only
three months remain - unless there's been an agreement to extend it.
1,700 UK Pounds was no bargain, IMHO, yet their follow-up effort to
ensure performance appears to be half hearted at best.

You've lost use of the car for two months - or was a free loaner offered?

If this were my car I'd put this written proposal on the shop's table:

The warranty be extended to add the car's "down time" since the June
failure.
The shop has two weeks to fix it or refund your money.

If it's not fixed or money refunded it will be taken to a M-B dealer for
repair and the shop sued for the repair cost.

Unfortunately such knotty problems are like quicksand, the longer they
go on the worst it gets. It's now time to be firm and extricate
yourself. Remember, this shop was willing to take on this complicated
repair - they weren't doing you a favor - it was business. They failed -
so far - and don't have the ability or willingness to perform at this
level. They don't like it either, looking at your car laying there day
after day, and would also like it to be gone - one way or another.