Thanks for the comments.
I researched and bought an "Electronize" FR15HVR-AN speed controller
from the extremely helpful guys in Sutton Coldfield.
The boat is weighing in at about 10lbs all up, and running a 45mm prop.
The ESC comes with some neat features, such as adjustable speed range, variable frequency range, and preset neutral, and I will be starting off very cautiously, I'll probably turn the throttle speed trim down after set up, and tape it there!!
And if all else fails, I can invest in two slightly heavier 6 volt lead/acid batteries
I may not make Chasewater this weekend after all, but maybe the one after.
Thanks again for the help.