air conditioning

hi i have a rover 623gsi 1996, i have just gassed it up and i have a leak on the mounting block at the bottom of the condenser, it is two halves and as a bolt fastening holding it secure on the right and on the left were the gas comes through there is a thread with an o ring type seal. i was wondering what i could do to fix this leak as everthing looks ok.. should there be a small gasket in between the two mountings anyone with any ideas ??? pleeeze help if u can thanxs stef623