CO2 was about 1 bubble per second or so, ph was about 6.5. Although the
test kit I have doesn't really measure that accurately in terms of range
my ph indicator I have permanently setup showed nothing untoward.

You said you increased CO2 - what was the pH before Saturday? (i.e. how
quickly did you change the CO2/pH?)

pH 6.5 too low to get much effective buffering from just KH - if you're
injecting CO2 by bubble count vs. pH controller, you might possibly have
gotten a big pH swing (especially when the plants stopped
photosynthesizing). I'm guessing in the dark, but a large, quick pH
swing could have killed off at least a portion of the bacteria in the
filter. The extra organics thus released could cause a bacterial bloom
of other, more pH tolerant bacteria, using up oxygen.

My tap
water has always been ok bar a high level of phosphate, which I use
rowaphos to remove, I'm thinking the rowaphos suddenly released all the
phosphate it had stored up due to it being totally saturated and used
up, I've heard that can happen.

So is the rowaphos actually in your tank? If so, a large pH change might
(I'm not familiar with the product) cause it to change it's phosphate
absorbing properties and release the phosphate. (Although I can't think
off the top of my head how a phosphate spike would deplete oxygen,
unless there was an algae bloom - was the tank white cloudy or green
cloudy?)