Ford Fiesta Mk 6 air conditioning

[CI80]

The air conditioning in my Fiesta Mk 6 1.4 LX doesn't work. When it was purchased the dealer informed me that this was the case and it needed more than re-gassing. Does anyone know what the likely cause would be and how easy it would be to repair? I know that when the air con isn’t used the seals perish and the system leaks. Can the whole pipe system be replaced by a home mechanic? Are there any safety issues with this job? Thanks