Have a dual unit with a separate stand-alone fridge and freezer units

The fridge is excellent.
The problem is the freezer unit >> an AEG / Electrolux, model A75248-GA1.
When switched on the digital display shows the temp. and the fan / compressor (?) is making an appropriate noise - but the temp stays at ambient.

Other engineers looked at, and consider refrigerant has leaked out.
Claimed compressor and fan working OK.

I live in Safron Walden, Essex.

Any advice / guidance / contacts to repair would be greatly appreciated.
Alternatively, any advice, etc, about sealants that could be used to repair also greatly appreciated.

HELP - it is a lovely unit, do not want to scrap.

Thank you and Regards

Roger Powell, May 2012