creda 26250E freezer section not freezing

My ten year old in built Creda 26250E fridge freezer has stopped working correctly. The fridge section appears to work but I have to have the thermostat in there turned up to the coldest setting in order to get the freezer to freeze and even then I think only the top section appears to get as cold as it should.
I cannot see any obvious defect in the door seal and the motor/pump works when I turn the appliance on and off or turn the thermostat up.
Do you suppose it’s lost gas or could just the thermostat be to blame?
If it is terminal can any decent substitute be recomended?
Thanks.