Many thanks,
Had a minor issue with heater hose last weekend which lead to the question. Have filled the car and it did self bleed, which was good, ow holding temperature well, bypassed the heater so the small hose outlet from the water pump goes straight into the thermostat housing (these are different diameters?). I don't think the vet needs any heating, any time of the year, the internal temperature always is warm.
Keen to install the hole to the thermostat or buy one pre done, in the near future.
Many thanks to all for responses.
Lefty