Credit Card not accepted

I have set up my cart to accept credit cards and debit cards.

I have successfully received a payment by Visa card, and have put through a dummy transaction with my debit card.

BUT I cannot process my own credit card!

I have a Mastercard, and after entering the details I an sent back to checkout step 2 with the message:

"The credit card number starting with was not entered correctly, or we do not accept that kind of card. Please try again or use another credit card.[cc]"

I have 2 questions.

1. Why can I not process my Mastercard?

2. In the error message received, the start of the credit card number is not shown. Why not?

1. Admin->Configuration->Credit Cards ... is Mastercard enabled?
2. Which payment modules do you have enabled? Admin->Modules->Payment
3. What version of Zen Cart are you using?
4. What addons have you installed?
5. What browser are you testing with?
6. Who is your hosting company?

1. Admin->Configuration->Credit Cards ... is Mastercard enabled?
Yes

2. Which payment modules do you have enabled? Admin->Modules->Payment
Credit Card
Paypal IPN
UK Debit & Credit Card

3. What version of Zen Cart are you using?
Zen Cart 1.3.0.2
Database Patch Level: 1.3.0.2

4. What addons have you installed?
uk cc 1.3.0.1 mod.zip by Kelvyn

5. What browser are you testing with?
Internet Explorer v6.0.2900.2180

6. Who is your hosting company?
Eclipse
https://www.eclipse.net.uk

Do you have MasterCard enabled?
Goto Admin>Configuration>Credit Cards
Make sure MasterCard has a value of "1".

Yes, Mastercard is enabled.

I haven't heard back on this one for nearly a month. Does that mean that no-one knows the solution?