As it happened the plane landed 15 minutes early, and I was quite quick off it. Some fast-paced walking in the terminal overtaking a few people and a well-staffed border control (and holding a UK passport) meant there was virtually no queue there. I got to baggage retrieval just as my bag came out, then walked straight out of the terminal just as a Picadilly line rail replacement bus arrived. Given I'd made good time, I decided to save some pennies and take it.
Bags loaded, got on, the bus left right away. We were out of the airport terminal 11 minutes after planned landing time! No traffic to be seen, so it was a very quick ride down the M4/A4 non-stop to Hammersmith, where I just got the tube to Euston (Pic/Vic, plenty of spare seats), and arrived with 40 minutes to spare.
A broken rail at Tring meant most trains were cancelled, but mine (the 10.45 to Glasgow Central) was running, and surprisingly not wedged. It made good time past Milton Keynes, especially considering the emergency engineering works.
All in all, it went pretty much like clockwork, and yet again a logical reason to justify taking the Heathrow Express evaded me.
Yes, it was T5 not T4. Oops!