Radiator blanking plugs?

The ones which don't have an off position normally come with a decorator's cap. You take off the TRV head and screw down the decorators cap instead, and this seals the valve off.

I did find a Pepsi screw bottle top fitted the outlet on some valves, but there's more than one size.

Can you not just stand the rad upside down ?

I don't think that's the problem! It's not coming out of the *radiator* but out of the bit of the valve to which the radiator was previously connected.

As others have said, the TRV should have been supplied with a 'decorators cap' which screws on, once you've removed the thermostatic head, and holds the pin firmly down. If you don't use that there is a danger - as you appear to have discovered - that if the temperature drops sufficiently the valve will open, even on its lowest setting, and cause a flood.

If all else fails, find something (e.g. one or two 5p pieces) which you can sandwich between the valve and the head, to make sure that the pin stays down under all circumstances.

Alternatively . . . Do your radiator tails have the equivalent of a compression joint where they connect to the valves? If so, you could make up blanking-off devices with the following ingredients for each one: Short length of 15mm copper pipe,

Your rad valve outlet would most likely be either three quarter inch or half inch bsp. You can pick up brass or iron caps from the plumbers merchant.

If you use brass then fit these with a rubber or fibre washer. If you use iron caps then it is best to wind ptfe tape on the threads.