PM wrote:
I need to drain down the cold water tank over the weekend. My CH is a gas
boiler on a basic on-or-off timer and feeds both the rads and the 3 year old
indirect HW cylinder.

If the HW cylinder empties, will it be damaged by running the CH?

The cylinder won't empty unless you specifically drain it via its drain
off point (normally on the inlet pipe at the bottom. Simply draining
the cold header tank above it will only allow the water level to drop
to the height of the top of the cylinder (actually to the lowest
Tee-joint above it).
And no, it sholdn't do any harm at all.