Hard to find, as most dealers websites are silent on the subject. I ended up buying a car with 1300 miles on it from the Holiways group in the North East. 3% loading for charging the whole amount to the AMEX. But with a discount on the screen price and nothing to part exchange the price was the same as the best "for cash" offers I had from other dealers on the same car.

Triggered the 241 early in my membership year and approx 20,000 miles too. Goes without saying the balance will get paid off in full when the statement arrives. A 260 mile round trip to pick it up but worth it as my local dealers were just not interested - spent more time trying to persuade me to take out Ford finance. I just walked.

The Gates network in the South (well - Stevenage down) also accept AMEX for the 3% load.

Regards
R3DV