some more numbers from over the last 24 hours
as drivesafe said turn on the ignition for a couple of min to get rid of any surface charge. voltage with both batteries connected went down to 12.26 volts. at which stage the traxide isolated the the Aux battery as it should. turn off ignition within 10 mins main battery which is now isolated (traxide light flashing) 12.38 volts. Aux battery (isolated) 12.80 volts i would assume that the surface charge would also have been taken off from the ignition being on and the 2 batteries joined.
1 hour later traxide still flashing so batteries are isolated main battery voltage is 12.80 and the Aux battery voltage is 12.87 volts.
in the morning same scenario as above both batteries isolated main battery is 12.76 volts and aux battery 12.85 volts. interestingly i turned the ignition on this morning after getting a reading of 12.76 volts and after i had turned the ignition on the main start battery (still isolated) when down to 11.85 volts car wouldn't start, battery stuffed.
based on the numbers alone before i turned on the ignition i thought the main battery is fine but after turning on the ignition i now know the main need to be replaced. what surprises me is how the battery voltage recovered but the it doesn't hold the power to start the car.