Yes, it was down to a possession on the up & down fast between Clapham Junction and Selhurst (possibly even further - I didn't go further than Selhurst on the Brighton line today). Lots going on: works to the canopy on the fast platforms at Selhurst; a new GSM-R mast going in between Wandsworth Common and Clapham Jct; a huge tree on the embankment at Streatham North Jct being taken down using a rail mounted cherry picker; and lots of other bits and pieces, hand grinding, other tree works etc.
I don't think longer distance services were calling at Selhurst or Steatham Common - would probably have made finding paths even more difficult, even with GatEx running a 2tph service (rather than the usual 4tph) and no East Grinsteads (some coastal services were calling additionally at Purley for a replacement bus to Oxted as the Grinstead branch was also closed).
All the trains were packed too, just like rush hour. I suppose this was partly down to football and the marathon, but there seemed to be a lot of families out enjoying the sunshine (platform 15 at Clapham Jct was very busy all day).