I am suffering from some very severe registration problems on this unit, the colours are at least 3-4mm out from each other at even intervals. Where on the machine do I set this up and make adjustments?

How many copies have you put on this 3220's ITB (transfer belt)?

After ~500,000 copies, the transfer belt often begins to slip and causes an offset registration issue. Also when toner works its way under the belt causing it to slip on the rolls. If you're near that copy count, it's probably time to replace or rebuild your ITB assembly.

I have installed a new ITB that I had on the shelf, its running perfectly now. I also have a Brand New transfer belt in a box, is it easy to install this onto the old assembly? Are there rollers etc that need replaced at the same time or do they all just need a good clean.

Being careful and following washer order carefully for reassembly, it still takes under an hour -- I'd definitely give cleaning and the new belt a try if you can do it yourself.

If a tech had to be called at $100+/hour, I'd lean towards replacing rollers that have had a long life already.