Thanks for the replies and here's an update which may be of use to others. In the end after a bit of research I ended up parking on a private driveway about 10 minutes walk from the stadium. I used the justpark dot com website, chose a location and paid my £13. We turned up at 4pm, had a bite to eat, watched the gig (excellent by the way) and got back to the car at about 10:45pm. The traffic leaving the area was a bit slow for 10 minutes but not unbearable. I chose this method because
a) I read some horrendous reviews of cars gridlocked for over 2 hours in the red, yellow, gold official car parks
b) the cost of parking and riding a couple of stops away on the tube would have been dearer (and less convenient) than the driveway option
Lastly, I must say I was impressed with the new Wembley. The last time I was there (Live Aid 1985) was in the old stadium, which had all the charm of a public convenience.