Honda Civic Hatch A/c not working

I have a UK 2001 Honda Civic Hatchback, the air conditioning no longer
works, the light comes on, but the air doesn't cool at all. To be honest it
has never been that brilliant even in our cool summers here in the UK, but
as I said now does nothing at all. I know nothing about A/C, what could the
fault be? I would rather be forewarned, I suspect the dealer will say, "it
needs a new unit that will be £400 plus labour sir.... "

Or can these units be readily serviced and repaired cheaply?

Any help or comments would be appreciated.


Patrick
UK