RE this and the related post.

The 17.51 from London was first posted mid-afternoon as an Adelante substitution and then cancelled, then at one point the website suggested it would be started from Reading as a Turbo - hence the reference to 3 carriages on the 20.58, but nothing went past Oxford, whatever happened, so it was the 18.21 for me, though the extra stop didn't cause a delay. We even enjoyed a rare high-speed run into the platform at Moreton-in-Marsh - pleasant change from crawling in after being brought to a standstill at the outer home signal on the edge of town, which seems to happen rather a lot of the time, especially to the 17.51.