Found the info I needed, here it is:
'Raddle is a colouring agent smeared on the ram’s lower chest between his front legs. It’s usually sold as a powder, which can be mixed with a little vegetable oil'.
I've seen the tubs for sale online, so, I may take this route. I think getting the goo on him will be just as easy/difficult (I'm thinking easier) as trying to get a harness on him, but without the worry of him getting rubbed or caught up on something in it.
Hope I remember to put the latex gloves on first, otherwise I'll be stained up to my elbow and I won't half get some funny looks doing the school run!
