ebay and Transcom

Dear Forum members,

I've had a look through posts in the forum, however, haven't found anything that can answer my question - I'll set the background and then give you my question, and hopefully somebody can help me

In April I sold a product on ebay, and I had 17 pounds worth of sellers fees. Then I moved to Germany for 3 months. My sellers fees were not settled until I moved back to England. The payment was late, by three months, but ebay still got their money (and even sent me an automated thank you!).

Then a week later, a company called Transcom, started calling me on a daily basis saying I owed them 17 pounds + 8 pounds of administrative fees. I said I had already paid ebay the 17 pounds...

Then they told me "Yes, ok, so now all you need to pay is the 8 pounds of charges required by us".

My question - if I do not pay the 8 pounds is this legal? Am I breaking the law by not paying the 8 pounds? Do they have the right to ask for this money? If i don't pay the money can Transcom add anything to my credit history?

Basically my main question is: are transcom able to obliterate my credit history with this? It's something that's stressing me, yet I don't want to pay such a vicious company 8 pounds.

Thanks for your time