I can't immediately find details of the case I was referring to. However, in general, the store has to justify its claim. £125 seems a hefty sum for two bottles of tonic even allowing for administration and investigation (of which there could not have been very much). Of course DWF (who seem to rely heavily on the CR scheme for their income) will want their cut. It is scarcely your fault that Tesco have seen fit to employ them, especially when you have contacted the store directly in an effort to settle the matter. To enforce payment the matter will have to be heard in the county Court and it may be that Tesco (or DWF) will not pursue the matter that far. However, the Citizens' Advice Bureau will be your best bet as a start. They will have knowledge of what happens in your local area and can advise you on the best way forward.