Thank you everyone - very helpful information.

I need to get to T5 and will be driving from East Kent, which is why I had thought Elizabeth Line could be worth trying to avoid any potential traffic on the M25... however looking at the above it looks as though that may be affected by the strikes too. And it looks as though the Picadilly line won't be running to Heathrow next weekend either!

Will be looking to park for the best part of a week - airport parking with shuttle bus was circa £150 vs £40 at one of the nearby stations