The company inolved is CABOT FINANCIAL, helpful responses only please.

If a company buys a debt, then that debt becomes unenforceable, however if the company goes bankrupt and someone takes over then the debt passes on to the people who have taken over. I had to look this up because I had a couple of letters from Lowell Group/Lowell Portfolio/RED (all the same company) then I saw Lowell Group on Watchdog, they buy old debts, most of which have already been cleared and try and get the money off you.
If you request someone doesn't ring you, they're supposed to not ring you, but to make sure, tell them to take your number off their system.
If the debt is in dispute (and it was the original company.) then they have to prove you owe the debt, but as this is a bought debt which is now unenforceable, there's not a lot they can do.
I'm not sure about the charges bit but I know a place that could help with most your questions (Martins Money Tips websibe)
Companies might not be ringing before 9am that they are aware of because of the time zone difference in India, where most of these people are based.
Ahh Now you said over six years ago, which means it's oh what's the word, if a debt is over 6 years it's not valid any more anyway, unless you have acknowledged the debt since.
Companies such as Cabot Financial, Lowell Group, Lowell Portfolio and Red buy old debts for £20 or some ridiculous amount and then try and recover the rest from people who don't know their rights.
If you either just ignore it, but then you'll keep getting letters etc. Or send a letter saying "I do not acknowledge this debt" then you have proof at least that you don't acknowledge it.
If they continue you might need to contact the FSA and then if they don't help, then it's Watchdog time.
(I watch and read lots about this kind of thing)

...Hi dorypanda, If you live in the states, write a letter to the company, keep a copy, request that they only contact you by letter, that they not call you on the telephone, it's called "cease and desist". They have to honor that. A company is allowed to purchase the debts of another company and collect the debt or a percentage of the debt, such as a settlement on the debt. If there is a dispute with the original company, it probably would have to be worked out with the new company, as your account would be with them. It you don't believe you owe the debt and can't work it out, do not, do not start to pay them, as the statue of limitations starts all over again fro the time you start paying them. Keep copies of any correspondence to them and from them, as well as checks or payments that you have made towards the original debt. Remember, if the bill is in dispute, don't pay it. Hope this helps you in some way. Good luck.

I just gave a long answer but that just disappeared. Basically, buying debts is legal but they can't force you to pay anything if it's in dispute. As for the time, I think there is a certain time before which they can call but I think it might be 8 rather than 9.