Zanussi ZX99/5SI frost build-up

I have a Zanussi ZX99/5SI frost free fridge freezer, and it seems to be suffering from frost build-up. I’ve read a number of other posts on this site relating to these machines, and so started by defrosting it for 48 hours. After this, it worked fine again for about 2 weeks, but the fridge is now starting to warm up again. If I turn on the fast freeze button, this maintains the freezer temperature (until it switches off automatically after about 24 hours), but the fridge stays warm. When it was warm last time (it hasn’t fully warmed up yet), every once in a while there was a repeated clicking noise – about 5 or 6 clicks in a row that sounded like something trying to turn itself on but failing. I guess its something relating to the defrosting of the cooler – the heater not turning on. Any ideas what the clicking might be from, and whether it’s likely to be fixable?

The fridge is about 32 months old, and I really can’t afford to replace it with a new one! (it was our wedding present!)