Hi filthy, first check out if they have a complaints procedure and follow it (if they have one)
Not too sure if a template will help as every complaint is different and dependant on the company you are complaining to.
I used to answer complaint letter in my old job so hopefully these tips will help:
If you can get the name of the person who deals with them, send it directly to them, (possibly follow up with a phone call to confirm they have received it)
State the nature of you complaint briefly in the first para.
Expand on it in the following paragraphs, listing (if possible) all dates, names, of when and who you have previously contacted.
Try to keep the letter relatively short (page and half of A4) but don't skimp on the details.
End by saying that you expect a reply by dd/mm/yy (or within xx days if it states it on their complaints procedures. If no reply is received follow it up, keep copies of all letters, brief transcripts etc of all telephone calls.
Finally good luck as if it is to either NTL or Virgin you may need it.