The RailAir bus goes from Stop 9 outside the arrivals levels at T5, it's non-stop from T5 to Reading, I've used it a lot over the years but have been caught out occasionally by the M4 being a car-park and then watching the tail lights of my train dissapearing off the end of the platform at Reading. On the other hand I was once held prisoner on the Hex coming into Paddington and they couldn't get the doors open, I watched my last train to the west go out and had to go back to Heathrow to rent a car to get home.

Beware that RailAir is only every 30 minutes at some times of the day, Also if you do go to Reading make sure that your train from London stops to pick-up at Reading. A few don't stop at all but that is very few.

To me it seems a long way round to to go back to Paddintgon to come out again espcially from T5, but the M4 is the great unknown. Mind you seeing that 3 of my recent train trips to Reading or Paddington have gone very belly up I'm not sure how reliable the railway is compared with the M4!

Hmmm not sure about this, certainly in my neck of the woods GWR get really quite stroppy about people joining trains late or getting off before their ticketed stop. Round here that's because on the evening trains out of Paddington it is often a lot cheaper to buy a ticket to somewhere in Devon and Cornwall and get off before Exeter. They don't like that! But then maybe they should sort the crazy ticketing system out and people wouldn't try it.