So once the room with the thermostat is up to temp, the rest of the
house 'loses' it's heating until the thermostat closes again ?

Yes. Either put it in the coldest room so the rest of the house is already
up to temperature before it closes off, or put it in the room you are likely
to be using (i.e. lounge) so you don't care that the rest may not be as
accurately controlled.

Much better still, use subzoning and have different programmable thermostats
for each zone. I have zones for upstairs (bedrooms), downstairs
(receptions), kitchen and (soon) conservatory. This way each can have a
different temperature profile over time to reflect usage patterns and each
follow more accurately the desired temperature. The thermostats are placed
in the most used (or only) room in each zone.

Christian.