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READ THIS FIRST - Private Parking Charges Forum guideWith regards to your proposed response, only threaten to counterclaim if you are actually going to do so. We don't have many details yet, but based on what we do know I personally wouldn't recommend counter claiming.
With regards to claiming 'harassment' - their first defence to this is likely to be 'we have been writing to YellowFellow for over two years trying to resolve the matter, and he has ignored us, we had no option but to go to court'. You may face an uphill battle to argue that you have been harassed for money you say you do not owe, if you have taken no steps to point out to the alleged creditor why you do not owe the money.
The other (arguably more 'tactical') argument against counter claiming is that it potentially increases the chances of the case going all the way to a hearing. If they realise they've got no hope with their claim, they may discontinue. If they have to turn up to court anyway to defend a counterclaim, then they may as well send someone to argue the initial claim, too. This can work both ways, in that they might agree to drop their claim if you drop your counter, but this of course would depend on the strength of each sides' cases.
A final initial point that may sound pedantic, but isn't... You refer throughout to "we" - who are Moorside writing to? If it is an individual, it is that person who will be sued, and from a legal standpoint there is no "we".