Managed to find some time to return to this and found the Fusiliers Association site with some great pictures of the Assault Pioneers. The webmaster there kindly gave permission to show some of the pictures on here which seem to confirm that a plated white metal and/or chromed badge was worn by the Pioneers on the apron at one time, at least by 2RRF and possibly another battalion. The photos are pretty low res, the early ones presumably being scanned from prints, but at least each time I go back to look at them, they seem to confirm my first impression. It would still be good to have a confirmation from any of the RRF guys that this was indeed the case. The first is a pic from here
http://www.fusiliers-association.co....1/Berlin81.htm
of a 2RRF Pioneer in Berlin dated 81-83, handily standing next to some of the family silver, which seems to show a GM badge on the cap and WM on the apron. This would fit with the timescale of the cast Indian or Pakistan made badge.