Hotpoint FFM97 Freezer not getting cold only reaching minus 7

Can anyone help? My Hotpoint Fridge Freezer, the freezer bit is only reaching minus 7. This is despite the fridge holding it's temperature at 2 degrees. It's the frost free type and I have taken the back of the freezer off and defrosted the ice build up around the element, the top left hand corner. But it is still not gettting any lower than minus 7, despite being set for minus 18. Is there anything that can be done. It seems to be making all the right sounds, fans whirring, compressor working. Any guesses anyone? It's about 7 years old.

try regassing it

Thanks tonto33. Any idea if this is a feasible cost repair? Rough estimate maybe?

we charge from 70 QUID