Samsung21jlsj – just freezer not cooling

Realise many problems with these fridge-freezers and have read many threads. The freezer side has recently stopped working and lost temperature. I got an engineer out (Domex) who checked the fans etc for icing (all clear) and re-gassed it which meant the freezer chilled lightly (sometimes freezing ice for a week) but now (2 weeks later) struggles to get beneath 3/4oC. Meanwhile the fridge seems to work fine at a normal 4oC. The unit has becoming a bit noiser and appears to be working harder. Unit came with the house so have no warranty etc but was only a couple of years old (i guess early ’06, late 05). Replacement seems to be £1300. Domex say I could replace the evaporator in the freezer section but have no idea whether this will work – would cost 87+VAT (already paid 100 for a re-gas and check)

Is this worth doing or should i give up – what is the chance of the evaporator sorting the problem? Many thanks for any advice.