Interesting designation regarding the monetary amounts involved; I presume you have been converting to your home currency rather than this being a question about procedures in Devon England. Here I strongly recommend that you ignore the requests from money received from or on behalf of the store. It is called a Civil Demand and is a required first step for instituting a civil lawsuit for damages. The second step, namely filing a lawsuit will not be taken as it makes no economic sense for them to do so; however, sending out these demand letters to lots of people brings in some amount of money with almost no time, trouble, or expense involved. Ignore them and they will eventually stop, probably after a few months. It's a money-making scam.
This is a civil demand letter. It is really only due after the store gets a civil judgment against you. Since the stores virtually never pursue a suit, I tell clients to ignore these letters.
Michael L. Doyle
This is not intended as individual legal advice and there is no attorney client relationship established by this answer. It is advisable that you seek individualized legal assistance. This is not a substitute for hiring an attorney.