Anyway, this morning I plucked up courage, took her down to Black Park lake near Slough and chucked her in. She appeared to sit well in the water. I gently engaged the throttle and waved a tearful farewell as she moved off 20 yards or so towards the middle of the lake. I took her round in a wide arc and back to port. I repeated this a couple of times and did some straight runs until the tiller mechanism jammed, fortunately she was within grabbing distance.
I was so relieved and pleased that she worked. She had a good turn of speed (nothing too dramatic) and seemed to steer well, keeping fairly upright.
There appeared to be little if any water ingress. The water cooling system for the ESC didn't work (I suspect kinks in the tube) but it didn't appear to be overheating. I'll attend to that and also the tiller mechanism before I take her out again.
Finally got my sea legs!
JB