It is mainly a filter clean that you could do yourself to be fair unless the fins of the cooler are all blocked with dust.
Remove the filters from under the front cover and either hoover them or bang them outside to get the dust off them. Then hoover the dust from around the fins and under the cover.
If it is not blowing cold air any more then the system will need re gassing and checking for gas leaks.
Expect to pay about £20 each maybe a bit more if they need re gassing.