Thanks for all the tips. I lost internet connection for a couple of weeks, hence no reply until now. I finally found a machine shop man who was willing to come out and have a look, (they are quite reluctant to leave the comfort of their shops!) He retapped the thread this morning (the old thread was damaged, explaining why quite so much oil was able to come out.)
I will give the hole a good clean out, change the oil and filter and then see if I can track down any of that thread sealant stuff. The man from the machine shop seemed to think it would work.
I am moderately hopeful that any big bits of swarf might have been expelled by the pressurised oil when it spurted out, rather than going in to the engine, though of course a few more will have been made when the hole was retapped.