on the M4 westbound about 2 miles before J17 chippenham there is usually a camera van on the bridge, just at the bottom of the hill before you go up to the junction.

and quite often there are vans situated on the bridges between J15 swindon - J17 chippenham, so keep an eye out there too

then as dan says, in the villages surrounding coombe there are usually a load of sly fuckers in hedges!