Very interesting thread to me! A friend of mine has bought the ex Eggenberger/Crichton/Petch Sierra, sorry Merkur XR4Ti and parts are being gathered to restore it to its original Gp.A specification. So nice to read some history about it here.
Here is the car in its Gp.A days, I am guessing this is probably Pukekohe January 1987:
And this is the car as it is now, ba$tardised into a "Thundersaloon":
Conrad