This dry weather break was possible on Saturday, so the photographs show the sequence.
1. The manhole bailed out till the water is below any of the working pipes.
2. Some bubble wrap floated on top of the pipes and water, so that if rainwater does get in, the water will get to a steady state temperature and lose less of its heat to the cold air.
3. The edges of the manhole and cover were well cleaned and allowed to dry. Finally, I used a sealant gun to go around the entire crack very very slowly and carefully, pumping sealant into the crack.
After a few weeks, after some prolonged rain, perhaps, I will cut the sealant and lift the manhole cover to inspect.When it rains, I will watch it, and hope that water will pond slightly before it runs away down the slope.
If it has remained below the pipes, the problem is solved. If it has crept back up again, then its going to have to be something I live with, an undesirable system loss.
A trail of sealant laboriously squeezed into the crack. This will have to be done every time I want to lift the lid to inspect it. Will try not to do it too often.