Tiscali's official complaints scheme is CISAS so you'll probably have more luck contacting them than Ofcom, not that putting a complaint into Ofcom as well is a bad idea but they won't help with individual problems. It may also be worthwhile sending a message to 'Trading Standards'.
Note that if this is a debt collection letter and you can't settle the problem with the collectors immediately then it's probably better to pay it first and resolve the problem later. It may take awhile but at least your credit history won’t be at risk.