I found something interesting, and got the thing to work much better.
Power to the Esp module comes from a 240vac to 5v smps module in a screened case, then that goes through about 6" of stout 24/0.2mm cable.
But its obviously not good enough, I soldered a 2200u cap directly across the 5v in on the Esp module.
The module now connects much better, data download is a lot quicker, and the readings when the page loads are either spot on or slightly high, about 10%.
So these things are very power supply sensitive, and it appears that they dont so much crash or not work with bad power, rather performance is degraded, I wasnt expecting that, thinking it would just work or not work.