There may be no point in writing back to the DCA.... as they have said that they are passing it back to Halifax. That implies that they are not the legal owners of your account and could not take legal action against you in their own right anyway....
When it's passed back to Halifax... they will contact you regarding payment. A lot depends on how they contact you.... they might mention a F & F in their letter.... they may not. You will need to look at the S.A.R - (Subject Access Request) from Halifax in order to determine the amount of unlawful charges they have applied (if you have not already done so)... but, they may be willing to accept a low F & F from you when it's returned to them... which would be less than the amount outstanding after charges have been re-claimed, just to close the account down. In which case, there may be no point going after the charges....
They will not want to keep the account running forever.... and if you are unable to clear it.... they could be collecting for a long while, which I doubt they will want to do.... so an F & F is definately worth considering.
Have you an idea of the level of charges Halifax have applied ?
