Just found this postcard on a site that BG John pointed out. It has previously been captioned as dated 1909 on a Wiltshire local studies website. The wagon colours may of course be fanciful - but I'm posting the link here for future reference.
EDIT:
A few notes from a discussion abouth this with Miss P and Nick:
* The wagons appear to be 2-plankers, and the front one has dumb buffers. They don't seem to be GWR engineering wagons, as these appear dark/black in contemporary photos.
* The wagons may not be GWR. They could well be contractor's wagons or - even more likely - Bath stone wagons (common in that area). The dumb buffers also suggest this. This website has photos of stone wagons from neaby Middlehill station that appear to be very similar/identical: http://www.choghole.co.uk/PHOTOGRAPHSOLD.htm (see "Wharfs").
* The colouring of the wagons in the postcard may be false. The colourist may have got it wrong, or the colour rendering from scanning/digitally enhancing the photo may have changed the original shade. However, the colours of the lineside structures appear plausible for the GWR.
* If the wagons are GWR and the colour and 1909 date is correct, it suggests that GWR red on wagons could still be seen in 1909. If GWR red ended in 1904, it is not unlikely some would still exist in that colour. Or the colour might even still be painted at that date. See: http://www.gwr.org.uk/liverieswagonred.html
The next step would be to examine whether there were red PO stone wagons in the area.
But so far it does look more likely that they are stone wagons, not GWR.
Thanks to Nick and Miss P for their insights!