GrahameP: As imj implies, it should not be "too long" for the credit card company to be interested - you've apparently paid MicroDigital 250ukp and got nothing in return. Did the credit card company themselves actually tell you that?
From my experience, MicroDigital have normally returned people's deposits if telephoned regularly, or when presented with a deadline after which you will resort to a small claims court action. (Neither of these options need you to pay a solicitor to write a letter).
Registered letters requiring proof of delivery have apparently been turned away by MicroDigital in the past, so that route may just waste you time and money.
If you placed your order directly with MicroDigital, rather than through a dealer, you're fortunate in a way, because you don't have to put up with them "passing the buck" as to who is responsible - as I did.