You don't have to sign their form, send your own:
Ref: Your Offer of Settlement
Sort Code xx-xx-xx
Claim No: XXXXX in XXXX County Court
I acknowledge receipt of your letter date xx/xx/xx and your settlement offer of £XXX.
I accept your offer as full and final settlement only for this claim of bank charges made on my account between xx/xx/xx and xx/xx/xx(dates of first and last charge) However I will only accept an unconditional offer and will therefore not be signing your acceptance agreement
I accept this offer without prejudice and I reserve the right to make any further claims should you apply future charges that may be considered unlawful under common law or in violation of the Unfair Terms in Consumer Contracts Regulations 1999 or Unfair Contract Terms Act 1977.
I will be willing to withdraw my claim and will send a Notice of Discontinuance to the Court upon receipt of unconditional full settlement of my claim
I am also not prepared to agree to any confidentiality clauses you try to impose, unless of course your client wishes to make an offer of due consideration in addition to the amount of £xxxxx, in order to be afforded this privilege by myself.
I trust that you will find this arrangement acceptable.