@RJR_38 I rang Jaycotts in Chester and ordered an 'ankle' for a Janome Cat C machine - mc6600p etc.
It screws onto the shaft of my Cat D machine in the usual way. The only difference is that the bit that clips into the foot is narrower than the original and so fits the Cat C feet properly.
As long as I don't set the stitch width wider than 7mm its perfectly fine - I've marked the Cat C feet with a small blob of nail polish to remind me.
I use the following Cat C feet on my Cat D machine - concealed zip, pintuck, beading, applique, fringe, binding, rolled hem and ruffler - all perfect ly fine