As the best Chinese/b & q garage door opener finally committed suicide after 18 years of daily use( the welds holding the chain sprocket failed), I had to replace it. Fortunately I had a new spare Chamberlain unit in the garage waiting for me to get around to fitting it to the other garage door for the past 7 years,( I later decided that being a canopy door it would be a bad idea to fit it, as it stopped me putting a tall vehicle into the garage.) so I fitted the new one in the required different position. The hex fixing screws were too short, so I substituted some nice hex coach screws that I had in stock. Mistake, the head of the coach screws fouled the chain carrier. Oh well, a quick replace with some M5 screws solved that problem.
Tried setting the remote control function. Failed. So, checked the remote lithium battery volts---OK. These opener units have a remote control lockout function but this didn't seem to be the problem. The other possibility was the door infrared safety sensors, 770EML units which I hadn't changed, as I had bodged these to work with the previous door opener and they were the specified units for the new opener. These units are driven by 24V and generate 5+ volts of AC signal(measured with a DVM) if anyone is interested. So, all seemed well.
Anyway, decided to replace the batteries in the remote and check the signal output from the remote with a UHF signal strength indicator (Fringe electronics). There was a low level rf signal (50dBuV) around. Still didn't work. Inspiration then struck, checked the short circuit current available from the batteries. Only 30mA. Fished in the battery box for more batteries and found a couple which gave over 200mA and fitted those. Checked again with the signal strength meter, over 80dBuV.
Hope this might be of use to someone else.