Thanks for all your replies, general as well as specific.
TBW between Harpenden and Leagrave - does that mean the cross-overs both ends were usable as normal, with effectively temporary plain line between them?
Was it one block section for the whole 8 miles? If so, with a 25 minute transit time, that means max 2 TPH on each line, and any attempt to run more than that would be doomed to fail.
The transit time suggests trains had to stop at each signal and either speak to someone on the ground or to the signaller. Why is that necessary, when there is known to be no signalling throughout the 8 miles? If non-stop passage is allowed, then the transit time should be a little over 10 minutes, at 50 mph. If a stop at each signal is required, then there is little prospect of exceeding 50 mph, to answer RobMorel.
John Webb at St Albans reports that the turn-back siding was still in use on Wednesday afternoon. That doesn't fit with what I saw later in the week from the south - nothing running north of Blackfriars (from Sutton) / City Thameslink (from Brighton). Did GTR just give up and try to blame NR infrastructure damage far more than was needed?