OK, thanks for that.
You would appear to have a system fault, perhaps a blockage.
I can only suggest turning the appliance off and fully defrosting it
Once fully defrosted (allow a rest period of at least 3 hours for the gas/oil to settle in the compressor) turn it back on and see if it starts cooling down properly.
If it doesn't and you still only get frosting/ice on certain parts of the evaporator I'm afraid you would need to get a refrigeration engineer to come in and check the system.
Unfortunately there is no diy repair for a system fault.
Give the defrost a go first, it may save a big bill.
The worst case scenario is a compressor change and a regas.
This would be expensive but you may want to get a quote from Hotpoint too.
They have a fixed price repair scheme. Give them a call and see how much they want and what options they offer.
Other than that, for an independent repairer you could be looking at £150 plus for a repair.
Hope this is of some help
Let me know how it goes or if you need more help