It's worth a look at yourself first if you can work a ratchet set, as most garages will charge you at least £30ph labour.
If it is just a perished seal and you take it to a garage it's a lot of wasted money, as you can bet they will charge you a minimum of 2hrs labour +vat.
Taking the bumper off isn't a hard job at all. Just take your time getting it off, then get someone (even the missus can do this) to help you put it back on so you don't drop it and scratch your paint.
It can throw a fault code for the pressure switch if the gas is too low, so I wouldn't worry about that just yet. It sounds as though it's either the seal on the pipe going into the condenser, or the condenser it self.
Personally I would opt to have a look myself first, as the money you would spend on labour will most likely pay for the parts your going to need, but it depends on if you feel confident to have a go yourself.