to check the thermostat,
drain the coolant,
remove the top hose from the thermostat housing,
unbolt and remove the thermostat.
now when its removed and u inspect it while its below 82c, it should be fully shut with no visual opening at all.
if u can see its partially open when cold, then its seized open and needs replacing.
if it is fully closed when cold, then next is to test if the working temp is still 82c.
look at the video in this post as reference to the procedure
http://micra.org.uk/threads/pollymobiles-rebuild.35251/page-95#post-633626
put the thermostat in a pan of water,
bring to a boil 100c and it should be fully open,
turn hob off and keep an eye on the opening as the thermometer reading slowly cools down.
by the time the reading reaches 82c it should be fully closed.
if its still open at 82c then replace it.