You're right it is a huge difference when claiming contractual/compound interest, but extreme caution should be urged if planning to do this, as there doesn't appear to be any legal basis for doing it. Please research thoroughly if planning to go down this route to ensure you understands the disadvantages, as well as the more alluring advantages.
For those who feel it is an absolute must, I suggest you make sure that the claim is under £5,000
including the interest. That way, if the worst happens and the case is struck out, they won't leave themselves exposed to paying costs, as well as losing their claim, as it's in the small claims court.
This issue is discussed at length on many other threads, so please look for those, or start a new thread in the appropriate forum rather than debate it here on the charges guide thread. I just wanted to make the point as it's raised in the above post.
Thank you very much for the information by the way.

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Are Bank Charges Unfair?
You bet they are!
Alliance & Leicester Moneyclaim issued 20/1/07 £225.50 full settlement received 29 January 2007 Smile £1,075.50 + interest Email request for payment 24/5/06 received £1,000.50 14/7/06 + £20 30/7/06 Yorkshire Bank Moneyclaim issued 21/6/06 £4,489.39 full settlement received 26 January 2007
Advice & opinions given by Caro are personal, are not endorsed by Consumer Action Group or Bank Action Group, and are offered informally, without prejudice & without liability. Your decisions and actions are your own, and should you be in any doubt, you are advised to seek the opinion of a qualified professional.