The failure appears to have been either 2J21 0702 Hertford North to Moorgate
http://www.realtimetrains.co.uk/train/G13146/2019/02/07/advanced or 2J23 0705 Watton-at-Stone to Moorgate
http://www.realtimetrains.co.uk/train/G13155/2019/02/07/advanced. Real Time Trains has the latter terminated at Alexandra Palace due to a traction fault. One or other of these appears to have moved on to King's Cross having been held in Finsbury Park platform 1 for a time, by this time as 5G26 - the next working of one of the above trains would have been 2G26 0810 Moorgate to Gordon Hill which was cancelled.
Inevitably this resulted in lots of short / no notice platform alterations and congestion as hundreds of people tried to get across to Thameslink core bound trains on platform 4. Fortunately for me, that included the onward driver for 9S13 0754 Cambridge - Brighton! 9S13 was given skip stops between Haywards Heath and Brighton despite being only 9 minutes late.
There was also an oddity whereby 2C13 0727 Cambridge - King's Cross more or less caught up the 2C11 the previous train from Cambridge (0648 from Cambridge North) at Finsbury Park and managed to overtake the 2Y15 0802 WGC - King's Cross...