Sorry to reply to my own thread but incase anyone comes across a similar issue, it just didnt seem to make sense with a minus offset.
A related thread was recommended and after some reading in there of a similar post something wasnt right so I stripped it back, tightened the bolts and repositioned the BLTouch as well as giving the bed a really good clean (it wasnt particularly dirty but perhaps it helped).
The thread - My printer is printing in the air (bltouch issue)
Whatever was wrong seemed to always be reading a huge dip in the middle of the bed and im still unsure why as its a flat glass pane with 4 corner tightening screws to lower or heighten the bed. My bed is showing level under heightmap now (I also flipped it the other way around but I would have thought that would lead it to dip upwards, which it has slightly but far less than the initial downwards dip).