Thanks to you all for your replies, especially Nixt - you have saved me from making a time-consuming mistake: it hadn't occurred to me that when the HW cylinder stat is satisfied there would be no circulation in the towel rail - although it's obvious to me now! I'm going to leave it on the CH...
I was considering moving our heated towel rail in the bathroom from the CH to the primary HW circuit, so that we can have it on when the rest of the CH is off, but I read that the Building Regs (Section L) now either discourage or ban this practice. The towel rail is small (three rail Myson)...
I hope you brainy guys and gals out there can help me with this one. Our 3-way motorised valve (Honeywell V4073A-1039) sometimes doesn't go to the mid position (CH + HW), which means that occasionally we have no hot water in the morning, only heating. If I give it a light smack it then moves...