boiler cuts off at different temperatures for domestic hot water and radiant heating
First of all "because I was told" is a very bad reason to replace something.   Next, the house thermostat has nothing to do with this problem.   Finally, the hot water tank has nothing more than its own thermostat, so it can't control the boiler temperature.  

The high-cut temperature relay may broken or the boiler's thermostat is failing.  It's also quite possible that after replacing the expansion tank, you failed to completely bleed the heating system, leading to large air bubbles and quite possibly a failure of the boiler thermostat sensor to be able to read the temperature of the water at the location of the external thermometer you're reading.