"just remember, halfords are on a drive to try to take back some of the parts business from small local motor factors, so they have been instructing their staff to offer trade cards to 'enthusiastic amateurs' as well as trade."
This isn't right. We have recently had a bit of a telling off for giving out too many trade cards. Previously we would give them to any one with a work ID card for a company related to the automotive industry (office admin people etc) but now we HAVE to see some kind of documenation linking you to a mechanic or technician occupation.
Luckily (as you pointed out) this is simple with a PC and a printer
If your local store won't accept letter headed paper try printing off a business card with your name and any variation of the word "mechanic" on it. After all, who has business cards if they don't actually have a business?
