I am happy to report that having replaced the ESCs in the Robbe Police Boat, it was running perfectly yesterday.
I have issued the crew with strict instructions to avoid showing off by breaking ice in the future.

It seems that my enthusiasm for ice breaking was unfortunate, as one of the ESCs in my Robbe Police Boat has refused to run in reverse since. I suspect that trying to use the props to pull it back into the water when it got stuck on the ice has overloaded an ESC. The boat uses one for each of the two brushless motors and I use differential motor control for docking manoeuvres and for other modeller's boat rescues.
I had a message from a clubmate last night to ask if I had had any problems with the mix as I gave him the settings and he is using it in a town screw boat. He described the problems he is having and they are identical to those I am currently trying to cure. The set up in my boat has worked faultlessly for 3 or 4 years of regular sailing.
I plan to test and re-test the tx mix, the rx and each of the ESCs and motors again to try to find where the fault lies.

I arrived at the Oxford MBC's Thursday afternoon get together today with my rather well used Robbe Police Boat. There was only a small strip of water available due to ice, so I set about methodically breaking it up with the police boat - which still bears the scars of previous years' ice duty!
I managed to clear enough open water for the couple of yacht racers to sail their Fiestas, and the ice breaking was good fun.
I don't think the police boat is really heavy enough or sufficiently vertical at the bows to perform as an ice breaker, but it was more practical than using the light Fiestas for the job.
Every year I do this, and every year I plan to build a tug with a reinforced bow and with plenty of weight in it.
And every year, I completely forget about it until the pond is iced up again!