MrP, the M4 will be extremely busy, as will routes in towards Hounslow. I imagine that you are going to exit the M4 at junction 3 and travel south on the Parkway, the A312 and then turn left at the Waggoners roundabout and travel down the Bath Road to Hounslow West. I'm puzzled about the 400 space car park however, I didn't think it was that big but it is just before the station, on the left. The whole of Hounslow is jam packed at peak times, given its proximity to Heathrow and major road routes, if you can, aim to arrive well before 8am.
jj