My air con has a leak, not sure where to start looking .someone advised me that it would most definitely be the condenser. Any ideas how much this job would be or how to test or repair it 

Hello Chris,

DIY work on air conditioning systems is really impractical, and indeed shouldn’t be undertaken since the gas should be correctly exhausted and stored only with specialised equipment. 
Simple checks for leaks etc. is neither expensive or time consuming, and should be left to the specialists. 
Kind regards,

Gareth.