In case anyone's interested, we got to Victoria at 0944 (having been lucky with the Central line!), obviously missed the 0945, but there were no queues and we had a coach to ourselves on the 1000. After seeing my wife off at Gatwick, I caught the 1146 back to Victoria, for which there was a moderate queue; passengers weren't allowed onto the platform until the incoming train had arrived.
My trains, and the others I saw, seemed to be pretty much on time - probably not surprising as there were no others about! Passing through large but deserted stations en route was a bit eerie.
Thanks to those who replied earlier for their advice.