To conclude this, I used all the advice above, removed the sandwich plate and checked and tightened everything. I took the first blat of the year this evening with no apparent leaks. Thanks to Stu, Paul, SM25T and John for the advice. It was slightly awkward to line up the plate and re-thread the hex nut (26mm socket needed for this). It took a few goes for me to be happy that the seal was evenly positioned. Think Automotive tech dept couldn't say which seal I needed but when I took it out it looked good so cleaned it up and put it back. If anyone knows which one is required, it would be useful to know for the future, for all of us Apollo users. I did spot a minor gash in one of the oil hoses which run through the spinning alternator belt, where the belt had nicked it. I was able to re-position the hoses to run slightly more centrally through the belt. Still a hair raising position for them though!