Thank you all for your advice and valued information.
I finally opted for the Standard Horizon HX290, which floats, is waterproof for up to 30mins in 1.5m of water (IPX8). It cost £102, so was within my strained budget. Solid construction, and a decent squelch adjustment, rather than the Cobra's squelch on or off configuration.
I also learned you get what you pay for with drybags; a couple of cheap ones let water in, and a couple of not cheap at all let some in, so they are now relegated to camping use, which is what I suspect they were meant for
Only casualties were some sweets for emergency (I am diabetic) tissues and some paracord I carried as a matter of course.
My SIB is a 2.6m and has a 4hp Evinrude, so I suspect the person suggesting I let the motor idle whilst out of the water a while though my boat was somewhat bigger and perhaps not water-cooled. As it is a vintage OB, it takes a little while to start, but once going goes well, or at least , it did in the bucket, and around the bay when we managed to get it out there.
We plan to try it out on the canal, which may be easier to launch and board/alight, and i can then try my hand at rowing if we come to reeds etc. I understand that there would be a speed limit on a canal, is it a universal 3 or 5mph? We shall only potter along to check out the views and get to practice handling the boat anyway
before considering our next very fine weather inshore sea trial