Somehow managed to bag two tickets on the Amex presale, had a normal and incognito tab on the laptop and at about 7:58 decided to try on my phone too and got through instantly on that one at 8am!
Which confirms to me that it is fully random - super frustrating system. The O2/Artist presale was a bloodbath after - apparently 70k people in it - congrats to anyone that got through!
The only tip seems to be try on multiple browsers, devices, laptops etc because they all appear to count as separate instances with separate queue positions as far as I can tell - you'll then know instantly which is your most likely to get through, so can focus on that one.