Why is there a voltage between the mains ground and my radiator?

I measure 1VAC and 20mV DC between ground and my radiator.

I would consider that negligible. Very likely there is a connection somewhere between your radiator and ground. It might be a long path though, for example:

Radiator - pipes to central heating unit - cold water pipes (does your heater also provide warm tap water?) - cold water grounding connection.

In my house at the place where the cold water enters the house, the water pipe is grounded together with the mains ground.

This means your radiator probably is grounded only through a very long path. Due to the length of this path there are all kinds of opportunities for it to pick up a small voltage.

I recommend grounding your ESD mat through the mains earth. Do make sure however that there is a 1 Mohm (yes, 1 Mega-ohm) resistor between the ESD mat and the actual ground. This is needed for your safety.

Suppose you have one hand on the ESD mat and your other hand touches a mains live voltage, the current must somehow be limited for your safety. That's the function of that 1 Mohm resistor. If you bought a "proper" ESD mat + ground connector, this 1 M resistor will be present already. But to be sure, I would measure it to confirm. For ESD that 1 Mohm resistor makes no difference at all, charges can still flow to ground through it.

One reason for potential differences might be that your measuring device closes a ground loop.

Basically there is a conductive path formed in your house that encloses an area where there is a changing magnetic flux. Due to the ever-present 50Hz (60Hz in some areas) field this conductive path becomes an antenna for this signal, which will give voltages and currents of the order you are measuring.

This is why you must only have one connection to ground.

The ground current does not trip breakers, as differential breakers measure the current difference between the live and neutral wire, the ground wire is not taken into account.

The best location for the grounded mat is right where you work ;-) Its purpose is to prevent the build-up of static charge, and that it will do at any location as long as plugged in to the ground.