With the help of all your responses I'm getting a better understanding. The rad is indeed plumbed into two pipes emerging from the hot water tank, but I've now realised that these are NOT (as I previously thought) the pipes carrying the indirectly-heated tap water, but rather the pipes carrying...
We have a sealed CH system with separate programming for ground-floor radiators, first-floor radiators, and domestic hot water.
We recently had a very small radiator fitted in our very small downstairs toilet, and — presumably because it was most convenient for him — the plumber connected it to...