Thanks to all you lovely people for your replies!
I reassured myself that all the mucky horrible work was done - more on that. Took the filter housing off and first thing I noticed was that the manifold was not seated properly on the top of the grids. So, I've tightened that up enough to ensure that it stays put but isn't too tight.
I went to my "favourite" pool store and bought more lube and another "O" ring for the elbow (is that the right term - it's where the manifold goes onto the inlet pipe). Manifold slid on nicely, much better than yesterday and the old "O" ring was definitely stretched out.
Cleaned and lubed the filter housing "O" ring and tightened everything up.
Success! I carefully watched the water returns and when I added the DE, water stayed clear.
Pressure reading on the filter is now 10/11 psi. Does that sound right?
However, next thing is all the DE that went back into the water yesterday. The polaris is indeed sucking it in but the scrubby tail is also doing a good job of stirring it into the water, which now looks cloudy again. I've also had a load of leaves fall in and I've been scooping them out so that I don't have to empty the polaris bag every 2 minutes.
I'm guessing that some of the DE in the pool water will end up back in the filter? When I added the DE this morning I added slightly less ( 2 scoops) because I could see some of the DE from yesterday on the grids. What should I do next? I am slightly concerned that there will be too much DE on the grids after all this.
But anyway, no more meltdowns from me today and for that I am very grateful to you all.
AH!! The breather looks ok but a bit old if not worn. I guess it wouldn't do any harm to replace it?