I have re calibrated the external probe again (twice) using water at 0deg and 100 deg. Still getting a 1.5deg difference between the external probe and pill at 20deg. External probe is stuck to the outside of the fermzilla and covered in bubble wrap. Not sure what else I can do.
It's quite possible that you do have an actual 1.5 degree difference between the external probe and pill. Bubble wrap is good and it will help to get a much closer reading of the core temp in the fermenter but it's not to say that it's perfect which is why customers also use thermowells to get a true core reading in a fermenter.
Just out of interest what do you get on the probe if you move the probe to the top of the fridge and the bottom of the fridge. I would also suspect that you would get a different reading too.
For instance if you get your domesic fridge that you put your groceries in at home if you put 4 different probes into the same fridge in different corners you will probably also get different readings.