Well teh relay was missing so put a new one in but compressor still not switching on ( figure it needs recharge ) . Looking here it seems a part may be missing ?

 

Just looks like an over-pressure safety valve. It's a one way valve that lets pressure out of the system if it gets too high, rather than letting something explode. Usually those are one time use, once it has opened it has to be replaced. An AC shop will be able to tell.

Great thank you

If the system is low enough, the compressor won't even come on.