Thanks Robin. I'll check the oil lid, not thought of that despite it being a fairly routine part that failed on my old Leylands.
When you say remove everything in preparation for applying heat shrink (presumably layers of wrap), would you take it all out back to where the dipstick tube attaches to the engine? And then wrap over the existing cracked filler collar and its connection to the dipstick tube. If you have a recommendation for a specific heat shrink please add a link. I'm a keen and fairly competent DIYer (but better with timber and masonry), I've got an electric heat gun I mostly use for paint stripping. Would this heat shrink be suitable
https://hilltop-products.co.uk/heat-shrink-hot-melt-tape-with-adhesive-inner-hdv-25-metre-rolls.html or perhaps better to buy heat shrink black tubing? Thanks.