Daniel, after your trip, I'm sure you know more about CdPs than most of us (and me), but as McCrankpin said: one stamp entering a State part of the CdP Convention, and one stamp on the carnet when leaving. That means, two stamps per country (always an even number of stamps). UK customs should not stamp anything nor request the RAC anything, so I don't get it, so someone may be telling your something wrong.

A second thought: in case you had left one country without stamping the CdP, that state may request your deposit from RAC (if I once understood right, they have a year to do it), since they would have find the entry slip cut from your CdP, but not the departure slip (they periodically pair entry/exit slips and find missing ones). For them, that would imply that your car remained in their territory: sold or abandoned without having paid import taxes/fulfilled import duties... (or you left and did not stamp). This thread tells about getting the departure stamp on you CdP after fulfilling the import duties in Iran... (this is funny, I just saw you actually answered the thread!!!)

You can show that they are wrong, that your car actually did abandon the country: a) with your carnet with both entry/exit stamps (I guess not your case, or RAC is getting dumb); or b) proving that your car is just plainly elsewhere out of their country. This last one, I am almost sure, not 100%, but almost.

How to work out option b)? I don't know, I'd ask first. But if I had to guess, I'd go this way: take your car to a Notary (or a Notary to your car) to state (give public faith) that the car with chassis nr, engine nr, plate nr is located in X place, all with notarized photos. That would show that the car is out of the territory of the country requesting the deposit and thus no import duties/custome taxes due that give right to ask for your deposit.

I insist, I may be wrong, cannot confirm 100%. Anyway, if things don't got any better with RAC, contact directly the AIT/FIA: Deborah Smith, [dsmith at fia.com], +41-(0)22-544.45.00 (Head of AIT & FIA Customs Affairs). Very friendly and helpful: I had an issue with Swiss TCS and one e-mail from her solved it all inmediately.

Good luck recovering what is yours! (and may I say again, amazing trip!)

Esteban

Edit: Read again, hold on, ADAC Germany issued your CdP (German Hilux?)? You returned the CdP or not? (1st vs 2nd paragraph)?