manufan - there really is no substitute for reading the small print and you are the only person with the small print available.
If you do a quick google for "MBNA balance transfer" you will see a lot of stories similar to yours where people have transfered a balance, said woohoo, spent more money, paid off some and then got a shock when the interest charge came in.
The normal way the MBNA offer works is as described above - low rate on transfer, high rate on new purchases, transfer balance gets paid off first, so new purchases attract interest straight away at high rate.
The scheme you describe (where you cannot use the card for the 1st 6 months) is different, but reading the small print will clarify this. I think it is actually that using the card within the 1st 6 months will cause the new purchases to attract interest at the higher rate.
I wish you luck in the small claims court. You might be on to something, but you *really* need to review the documentation and see if you signed something saying "I have read and understood the terms and conditions" and then check to see if the terms and conditions include the conditions you have fallen foul of.
I'm not trying to say that MBNA (or any institution) are allowed to stomp all over the customer, but if the conditions are laid out and not adhered to then they are allowed to add on charges as outlined.
z