Leaking diesel pump 1996 RR p38 DSE

Hi everyone in Landyzone land, I am not new to this site but it is my first time posting, I've been in touch with Datatek today and he suggests I throw my problem out with you guys, as the title suggests I have a leaking injector pump on my p38 diesel, got the seals to do the job and have stripped everything down to get access to the pump, top two seals no problem, but I think the 'O' ring at the front of the head may be leaking too, this is where the danger lies, if I bring the head out too far there is a chance the little shim on the cam mite drop, any advice on this would be most welcome.