The H/W feed to my indirect tank then goes to into a towel radiator in the bathroom (teed off before the header tank). The feed from this bathroom radiator then goes to the radiator return circuit. Is this a viable setup or should I get it changed?
I am certain that the hot feed to the towel radiator comes out of the top of the main H/W feed to the tank (teed of after the tank but before the open pipe into the header tank). It is hard to trace the return but it looks like it is going to the radiator return. I am sure someone set it up to have a summer radiator.
The H/W is gravity fed and the C/H is pumped. My concern is that contaminated water from the C/H circuit could get into the H/W circuit!.
You don't need to worry about any contamination of water as all the central heating and hot water heating circuits share the same water anyway. Inside the hot water cylinder, behind the two side connections, is a large heat exchanger coil round which the heating water circulates so the two waters are always kept entirely separate.
Your suggestion that the radiator being plumbed to the gravity feed side would make it work as a summer radiator does hold water (sorry).
My only concern would be is because the heating return is utilised, this crossover could, depending on exactly how the system is plumbed, cause a reduced or unexpected flow in the heating circuit thereby reducing its efficiency. You could also find that radiator doesn't heat up as much as the others.
If your system works all right, no harm is going to be caused by leaving the plumbing as is.