It looks like it is pretty easy to fit it over the head without removing anything to do so - so I did.
Then I noticed that it wasn't seating well. On further inspection I discovered there was a lot of melted filament (technically known as "gunge") stuck to the face of the heating block. I turned the heat back on, and tried to wipe it off without burning my fingers. I found that some slightly abrasive lens wipes were the most effective. They are impregnated with alcohol which might have helped - I hope that's not a bad thing.
FItted the boot and re-ran the TreeFrog - the one that failed with "Thermal Runaway" last time I ran it. This time I used absolutely bog-standard parameters supplied by Prusa for Generic PLA.
As usual, the temp sank on layer two. The temp for layer 2 is supposed to drop from 215 to 210. In fact it dropped to 205 and then recovered in about a minute. So far so good.
After each print quite a bit of gunge builds up around the sock hole and interferes with the next print. Seems important to clean the sock off every 2/3 prints.