Food poisoning stopped any progress over the last few days, today however I did manage to fit the circlip to the tank filler neck,
check that the filler cap lock located on the circlip and the key could be removed.
Finally I cut the base of the filler neck to allow the petrol into the tank leaving a “bridge” to retain the in-tank filter. The bridge ended up about half the width I had anticipated due to my complete inability to cut a straight line with a hacksaw, fortunately I can file a reasonably straight line.
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I also ordered some fine brass mesh to make the filter which I should be able to solder into a cup shape to sit on top of the bridge