See I had the same problem when I showed them my card did not recognise it and refused to give me trade club. This is because it is not a standard issue trade club card. Ours is a MIG Club card with trade club details on the bottom.
The way I sorted out my TC discount (with the ever helpful Hutchings Vauxhall in Treforest ) and phoned the two dealerships was as follows.
1. Ask "Do you accept Vauxhall Trade Club Discount"
2. Then give the clubs trade club account number 252660 and say the title should be MIG Performance Vauxhalls
3. Explain you're part of the club and show them your membership card with the TC details on the reverse as proof that you belong.
4. They will set up a cash account for you but need an address, use the clubs's address and not you're own.
Of course if they say no to number one you're screwed