Sort of not what I was wanting to hear although there's some positive in your comments.
Yes, body heating is what we were looking for, our north facing room that is used in the evening is often cool as most of the house benefits from thermal gain which doesn't get to the front. Also our daughter's flat, old stock, 3 outside walls and a large single glazed sash window could benefit from body heating as she works from home most of the time. We both use oil filled rads which work, sort of.
Although I thought infrared heated the objects in the room, bodies included, rather than the air as in convection heaters so the building structure warms up.
We might try a cheapy one to see how it works.