You've still got their original letter saying the closed the file and returned it?
Make a copy of that, plus the current demand, and send it to Cahoot with a letter headed 'official complaint' !
Send the 'bemused' letter to Moorcroft - http://www.consumeractiongroup.co.uk/forum/debt-collection-industry/175840-halifax-problem.html#post1900426
Call Consumer Direct and ask them to get someone from trading standards in your area to contact you, TS don't always deal direct with the public, and lodge a strong complaint with the OFT and FOS.
In general, make a fuss and make sure n