I can't believe that the advice given by somebody who woked for a debt collection agency is "Ignore them".
You should never totally ignore people threatning you with collection of monies owed.
Sure, if the debt is in dispute then dispute it.
Write the correct letters and get the right people involved.
Just sitting back and doing nothing gets your case to court quicker and once they have ruled it all becomes a lot more difficult.
Not entirey true, unless it is a court appointed bailiff, hired as the result of a lost case and refusal to pay the court, then they are merley invoices, these people have no power what-so-ever and rely purely on intimidation to recover cash, use lots of official looking letters with very strong wording.
They usually purchase debts from companies such as Gym's, mobile phone companies etc, for as little as 10p in the pound, so for an alleged debt of £100 they might pay £10 for.
If they succeed in recovering over a tenner from you, they are in profit, but we all know they demand vast sums of cash, and they rely on people being scared into paying. This is how they make money, and they make lots of it, as most people become intimidated and just pay.
I would advise just sending a single letter to them by recorded delivery, stating the dispute with the gym in this case.
Never speak to them on the phone, never engage in any comms unless written letter by recorded delivery.
These companies dont go to court, they just hound people until they cough up, if they had any kind of case then they would simply send a single letter, and then comence proceedings, but they do not, they just keep chipping away in an attempt to grind people down.