Although not strictly kosher this usually works

Dear Mr ..........
Despite numerous attempts at recovery your debt for our services remains outstanding,
This will be my final request at recovery personally.
If this debt is not cleared within 7 days I am left with no choice but to pass the debt onto a recovery agency, the £75 cost incurred by passing it on will be charged to your account.
If they fail to recover the debt they will take action via her majesty’s court service.
Please note: Interest will be charged from the date the debt became due as set out in the Late Payment of Commercial Debt Act 1998.
Additionally we also report any outstanding arrears with credit reference agencies 28 days after the date of this letter, which may affect your ability to obtain credit in the future.
Current outstanding debt £xxx
If you wish to discuss this matter personally please do not hesitate to contact me, Ignoring a debt does not make it go away.
Yours sincerely.