Our booking engine at tickets.railforums.co.uk (powered by TrainSplit) helps support the running of the forum with every ticket purchase! Find out more and ask any questions/give us feedback in this thread!
Looks like they’re going into the sidings and back out again to slot in between SE services.
GTR do sign via Blackheath as a diversionary route but there’s a block on the slows near St Johns which is presumably why they’ve curtailed them at Plumstead.