but is this usual for trains to arrive early on a Sunday night?
Basically, yes! There's plenty of slack on Sunday's between Reading and London as trains are all timed for when there's a 2-track possession leaving only 2-tracks open for part of the route. Obviously the expresses then have to fit in behind all the stopping trains, but if there's a Sunday (like today) when there were no possessions at all the trains can speed along unhindered.