Good afternoon my name is ***** ***** I will assist with your query today.
E4 signifies a failure with the integrated heater designed for the'self defrost' function, for the cooling elements and back panel.
This is clearly a hardware failure, I would suggest you arrange an engineer to site to strip the unit down and check the heater, it may be the heating element or the defrost sensor at fault. Which is probably why the appliance isnt keeping temperature properly.
If you provide you postcode I can recommend an engineer in your area.
Due to your proximity to London its looking around the 130-150 mark, which two thirds is labour costs.
I think I would be inclined to buy a replacement freezer such as the one here for £190.
If you do decide to get the unit repaired I would suggest you use Harris Refrigeration -(###) ###-####1582