First question: The only diagram I can find including a hydronic towel
heater has it attached to the CH system. But mine comes on when the
HW is on (even when the CH is off) so I guess it's connected to the
HW system. In this case how is HW pumped around the heater? I thought
only CH systems had a pump. My HW system is a mains-pressure unvented
Megaflo HW tank.

As far as I can see the h/w type is irrelevant. The rad will be fed before
any valves that go to h/w or c/h PRIMARY feed. This is usually done to
provide a bypass for the boiler/pump in case all valves are closed.
Also the theory is you want your bathroom/towel rad warm even in the
summer.

Thanks very much for all this. Can I be totally sure that I understand the
"feed" terminology please....
Ignoring towel rails for the moment, say you just have a boiler, a HW
cylinder and a set of normal room-heating CH rads. Then the pump that gets
the HW from the boiler pumps it into a 3-way valve that lets the HW go to
one or both (or neither - in which case you need the bypass) of:
(a) the "HW primary feed" - is this the technical term for the HW cylinder?
(b) the "CH primary feed" - is this the technical term for the bunch of CH
rads?

So then if you wanted to plumb in a towel rad that came on whenever the HW
was on, would you have to supply it by branching it from a point in the
pipework that lay between the 3-way valve and the HW cylinder?

Finally (sorry about all the questions, I've never tried anything like this
before!) when I put the emptied towel rail back on again, I just turn on the
HW pump and bleed at the top until all the air is out - simple as that?

Thanks again
Richard