I thought I'd better complete this thread to show my appreciation to those who took the trouble to reply and to help anyone else finding themselves in a similar situation.
After buying some quality 3mm drill bits, I finally managed to drill out the body of the bolt that snapped. This was the aft one of the two holding the complete lower section (gear housing and prop shaft) on to the unit.
Having got this far I though, what the hell, in for a penny, in for a pound and gingerly undid the other bolt. Typically this came off with no trouble at all. I then eased off this lower section which enabled me to give everything a good clean (it wasn't that bad and nothing appeared corroded).
The trickiest bit was reassembly. The long pipe that came away with the lower section must engage with the tube that can be seen when looking up the main section towards the engine. Make sure that these two pipes mate up as it's very easy to miss with the long pipe resting alongside the tube rather than in it. Before the lower section may be pushed home, there are two pipes coming from the engine end that must mate up correctly with the lower section. To do this you need to remove the large neoprene cap you'll see in this lower section. Using a piece of stiffish wire bent into a hook at one end and a lot of patience each of these pipes can be seated in turn via the hole now exposed by the removal of this cap. Take care especially with the pipe that carries the cooling water to the engine as there is a neoprene seal that you don't want to damage. Once everything is seated correctly, do up the two bolts holding this unit on, press the neoprene cap back on and bolt the prop shaft housing back on to this lower unit.
Having done all this I was now ready to add the lube oil. This time it was easy to pump the oil into the bottom hole until it came out of the top hole. Having sealed these holes with the bolts (top hole first before the pump is removed from the bottom hole), it was time to start the engine.
The plastic dustbins issued by the British local authorities are ideal for this. They are deep and the plastic rim is strong enough and just the right size for bolting the o/b on to. Having bolted the o/b onto the bin (remembering to empty it first
) I then added water to the correct level, prepared the engine for cold start, said a quick prayer and pulled the cord. Bingo, away she went, sweet as a nut.