Thanks Miss Wiggle, I was wondering if the pH drop might be a good sign, glad to hear it isn't all bad.
Waterdrop, I intend to do a water change down to the filter inlet pipe, based on your previous advice. That will be about a 70/75% water change.
Sounds good Stu, that's what I do when I do the change, although I must admit I power off the filter and shutoff the taps before refilling.
Obviously keeping the water above the mesh of the inlet pipe means you don't lose the syphon, and therefore can switch the filter straight back on again, and I close off the filter so that I can make sure the water is dechlor'ed and ideally back at 'tank temperature' before I start pumping it through the filter again (although to do this you'll obviously need to add some hot water when refilling).
I've refilled the tank with cold water only a couple of times, and then just opened up the filter once the tank was full again, and each time it knocked me back a good 2 or 3 days in the filter cycle, which I can only put down to the cold water (around 15 deg C), being passed through the filter until the tank heater got it back to temperature, (usually 12-18hrs). When I do my next change the kettle is going to be put to good use
Have your various heater troubles sorted themselves out now by the way?