A mate made mention of sucking the oil out of the tank which reminded me that, many moons ago, I had bought a "Pela 6000 oil extractor" for use on our Mazda Bongo. Saved scrawling around on the drive to get at the sump plug and then spilling oil all over the place.
Having dusted it off, it did a fine job of sucking every last drop of oil out of the tank - although it took a while.
I cut off the copper ring from the drain plug/filter and replaced it with a fairly chunky O ring out of my selection box, cleaned up the threads and fitted it and, Hey Presto - after refilling with oil, several hours later - no weep
Thanks again!
Brian