Have not got a camera and needless to say no mobile phone. In a few hours when I can get access to the boiler I will follow your suggestion and tell what I find. Unfortunately I assume I have blown the boiler thermostat while playing around, fortunately I have a couple more.
That is an idea, at least I could have a look and verify the valve type. Maybe it is a gate valve but it did feel like a tap valve as I felt I was compressing something when I shut the valves. Of course gate valves do break but you are aware all is not well as the operation becomes too easy...
My system is based around a Glowworm Ultimate 50ff some thirty years old. Over the years I have worked on it many times and always managed to solve problems, thus i have an extensive supply of parts. However after all this time finally the Grundfos pump gave up. I replaced it with a Grundfos...