This from a friend who works for Qinetiq:

Total urban myth, I'm afraid. Has been circulating for several months now but been given a recent revival.

Check out the official Thames Valley Police web site where you will find a categoric statement that the cameras on those M4 gantries are for traffic flow monitoring (and checking whether the signs are working!) and are not speed cameras of any variety. There are apparently other sites confirming this too, possibly including the manufacturer's!

SPECS cameras may be found in the M6 roadworks at Warrington and Knutsford (24), in Gloucestershire (Podsmead Road, Symour Road and Tuffley Avenue (12)), A616 Stocksbridge in S Yorkshire (14), Notts: A46 Cropwell Bishop, A52 Bingham, A610 Nuthill Road, B6004 Arnold Road, A6514 Notts Ring Road, Edwards Lane Arnold (60); and Northants A43 Lumbertubs way (8). But not on the M4.

However note there *are* apparently cameras (normal variety) in the road works at the M4/A34 junction @ Newbury. Apparently they are also fond of using speed camera vans on motorway bridges on the M4 in Wales. Near Magor services.

(We have a vehicles newsgroup on our intranet - it's a great source of info sometimes!)