I bought a hand pump, but couldn't get it to work, so resorted to traditional methods, i.e. bailing it out with a bucket!
Something squidgy like Grease or margarine or lard would not last and would make an awful mess. I am trying a dry solution first. I cleaned out the rim edge thoroughly, and have run a thick line of sealant around the edge, and laid a cut slice of bicycle inner-tube along the edge. This gives a dry surface to put the lid onto, and I hope the rubber will remain stuck to the sealant. I put the lid back on, hoping that it will mould the sealant and rubber into a profile that fits the manhole lid exactly.
It has rained almost all day, but I need about 3 days rain to be sure that it is working. Then, I will lift the lid and inspect.
It is a great comfort that there is no green or blue in there. The main system is no longer leaking, except at the new energy meter, and that is under control. I need a couple of hours and lots of PTFE tape to fix that.
Another idea would be to work something into the crack from above. I'm thinking of something non greasy - I'm thinking of getting Cat-Litter, watering it into a mush and working that into the crack. Most Cat-litter is Bentonite, and this seems to be the cheapest method of getting some.