Hi Rod,
having experienced (probably) this exact failure on my AMF first hand - are you certain that it is diesel coming out? There is a known oil leak location at the rear corner of the plastic rocker cover right next to the tandem pump and liquid coming from there often targets the same components underneath. If the engine/gearbox mount bushings are worn on the right hand side of the engine bay as you look from the front, the right hand end of the engine is slightly lower than the left. Consequently, a leaky gasket seal for the rocker cover will always leak oil from that corner down the back of the engine on that side, coating all of the coolant (heater matrix connections) pipework as well as the return to the coolant expansion tank.
In my case - the first issue was definitely diesel. It stank of diesel and made the coolant pipes there go very tacky so they needed to be replaced. I replaced the LUK original with a Bosch (more reliable but to fit a BHC-engine car you need an extra piece of EGR tubing to make it fit properly). Subsequently an oil leak was evident down the back of the engine - eventually traced to a combination of worn turbo (which was replaced last year fixing most of the leak) and also the leaking rocker cover (this year's problem, to be done in conjunction with replacement injectors).
Any more info will be helpful for those of us who have experienced any of these problems to help diagnose.
cheers, Robin