Thanks for your suggestion FLJ. I may well be purchasing one of your P94's yet.
The Eugen video is quite similar to what I get now.

Cutting the supply to one of the motors is not enough given any speed (the mixer stops one side now). I really need some "suck" to bring her round. I've figured out how to do it without cutting any wires or anything. I'm not overly fond of the present speed controllers anyway. They are little blue boxes with something like "orgasmatron" written on the side. It has to be said that they work fine, but they pulse the drive train creating quite a loud "knocking" noise. So if they do go up in a cloud of acrid smoke I'll be pretty philosophical about it.
I did suggest widening the canal where we sail from the present 15 yards up to about a mile. Thus not only curing my turning problems but also creating a true English channel experience for us with 1/24 scale boats. Sadly this proved to be a nonstarter when it was discovered that the handle on the club spade is currently broken.
The other option is to sail on the local river which is about a mile wide. Though I'm not sure how long this activity would last as the current is pretty damned strong, not to mention it has the world's second highest tidal range. On reflection, this might be my cheapest option for solving the problem.
I'll give it a go. If it works, I'll post a Eugenesque video. If not, I'll go quiet for a while, then buy one of your units, hoping, you'll have forgotten all about my clumsy amateur antics.
Pete