you might try looking at this US ebay item # 230355078952 it is called the egg-o-meter and it is an egg shaped thermometer probe designed specifically for monitoring egg (not air or substrate) temperatures inside of incubators, eggs have mass and are not necessarily the same temp as the air or substrate (admittedly the difference is negligible but if you want to remove any doubt) or if your just gadget curious, be sure to get the small size if you get one, if not i would place it ( a normal digital probe probe) half way in the substrate between two eggs. / if you cant find it by item # they probably have it at ; incubatorwarehouse.com /or it may even be available directly in the UK