Does anyone know a supplier of signs for putting on GWR coaling stages? I am looking for something like the one at Didcot that reads "Caution Engines must not pass this arch".

It does not need to be an exact match as these signs varied slightly across the GWR and were not completely standardised. I would be happy with etched plates, transfers or even vinyl stickers.

Any suggestions welcome.

For the driver to read that sign he would have had to, erm, pass the arch

That sign is on the approach to the coaling stage. The loco inside is a preservation "naughty". 

That is "Bonnie Prince Charlie" known by all at Didcot as BPC or the "boys pedal cycle" when I was working there - at about 17 tons, she weighed less than a loaded coal wagon! one trip down the branch line with a coach was like going to Plymouth on a Castle, you would start off with the water right at the top of the gauge glass, the fire almost falling out of the firehole and blowing off, and by the time you got to the other end there was very little left of anything, and the fireman would have to get it all back before we could go back to the other end! it made a day with BPC really exciting - wish I could do it again.

Regards,

Alex