Simply reply along the lines described at
https://www.justice.gov.uk/courts/procedure-rules/civil/rules/pd_pre-action_conduct#6.1,
the defendant responding within a reasonable time – 14 days in a straight forward case and no more than 3 months in a very complex one. The reply should include confirmation as to whether the claim is accepted and, if it is not accepted, the reasons why, together with an explanation as to which facts and parts of the claim are disputed
.
You are just following a process here, and your main points will come in your defence to the county court claim which will follow. Gladstones will pretty much ignore any details at this stage, so don't boil the ocean and keep your reply succinct.
PS You do not need to use their "reply form".
