How many times have you tried writing to them? Presumably this was a hotel complaint Have you tried taking it up direct with the hotel?

the company I actually booked the holiday through are yourholidays.com who act as an agent in organising each part of the holiday, and to whom I submitting the original complaint, they then forwarded it to Hotels4U as it was a complaint about the accomodation (or lack of), I recieved an acknowledgement e-mail (from yourholidays.com) on 5th October saying they had forwarded it to Hotels4U. I have had no other response despite sending 3 e-mails after 28 days had passed (ABTA Code of Conduct)

I'd send a recorded delivery letter to the company outlining your complaint.

Hi Glynis, send a recorded delivery letter to Hotels4U? and not my original booking agent?

yourholidays are part of the Hays group. It does seem to become very complicated when you use an agent who then uses a bed bank to book accommodation. Glynis is right. Send a recorded delivery letter to yourholidays but I would also send a copy to Hays Travel head office and explain what has happened re the lack of communication. Tell yourholidays this is what you are doing.
It is possible yourholidays are still chasing hotels4u but they should be back in touch with you explaining what they are doing to try to resolve your complaint and where they are upto with it.