The silicone treatment has worked a treat - it's still in there, and still keeping out the water and dirt. If I need to stip the engine out again - it'll just be a case of clean it up and put fresh silicone in - about as easy a fix as you can get.
re your other points - it's easy to slag off...
I put a little in the front of the casing too - but there was no ingress of dirt from this side so not really necessary.
Silicon is easy to pick off - so if it doesn't work, then there's no issues. Nothing ventured - nothing gained !!!
Having seen all the gunk and mud getting into the gap near the top of the casing on the drive side, I decided to strip the motor out and have a look. It was much worse than I thought - all of it congregates around the power lead input. I'm sure others may have come up with alternative fixes...