After thinking this through over and over, and wavering between a French supplier which may not have such a good after service reputation as Amazon, and Amazon UK, and, not getting any joy with the radio of the Manhattan, I finally bit the bullet and bought the Amazon one.
Sod never had such a good handle on his/her law. The day I finally ordered it I tried one last tactic to educate the current model. Instead of switching off at the mains each night and back on in the morning, I left both the mains and the Manhattan alone overnight, only returning it to Radio 4 beforehand, and then switching off the tv alone. Next morning, switch the tv on and touch nothing else, result, instant Radio 4 alive and kicking. 
Done that successfully for 3 mornings now, who would have thought it? But the fact remains that I had no need for such subterfuge over many years without any problems and, when switching from one channel to another now, the new one still goes to No Signal occasionally and has to be kicked up the arse by a quick flick to its neighbour and back again, so the writing might still be on the wall for the old decoder.
Thus, when it arrives this morning, I will not send the new one back, but keep it in reserve for the dreaded day. 
