You will see that Monday was particularly bad with Gatwick Express - 57% of its trains were delayed, whilst 26% were either more than 30 minutes late or cancelled.
I was also delayed considerably when coming up to London on Monday when my trains was cancelled and then subsequent ones delayed with the excuse given of a passenger taken ill on a train at Clapham Junction. This was given as the excuse for every delay in both directions on the London Brighton line for some time which seems to me as unfeasible, particularly given the size of Clapham Junction station.