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I dont understand why the refund didnt include the £100.
The company COULD argue that it is included in the refund.
Otherwise, send in the invoice for the £100 and make it clear that this has to be paid to ward off court action.
Actually the only thing you have to plead in a small claim is negligence and this isnt a case of that - so you dont have to say what bit of the law you are relying on.
I thik it may be this:
http://en.wikipedia.org/wiki/English_unjust_enrichment_law
but you dont have to plead it - all you do is put down the facts:
[ They made me pay £100 to gather evidence that they had overcharged }
The claimant should be the person who paid the £100
so long as he also had a loss
The first thing is to send in the bill and make it clear that you will go to small claims