This is some 'Hunter' pipe [if I recall correctly]. Very brittle stuff, I would avoid it at all costs. I have suggested to several customers it was all taken out and replaced with copper. In 1970's there was a copper shortage after a trade embargo on Rhodesia so other materials had to be found...
New one on me today. Heatslave 20/25 locking out. Changed nozzle, checked oil pressure and it ran ok. Left to run for a few mins and then disconnected my oil pressure gauge.
Boiler then wouldn't fire and went to lockout after 12 seconds but with no obvious sound of ignition. Decided to check for...
CT1 will be your friend here. I temporarily repaired a pin hole in a copper cylinder with it. After replacing the cylinder I couldn't get the CT1 off to look at the hole.
Your boiler has a frost stat which fires the burner if the temp inside the boiler casing gets below about 5°C. To stop if firing for long periods, a low-limit stat can be fitted to the return pipework in the boiler, so that when the return temp reaches 30°C it will turn the burner off.
A new filling loop shouldn't take too long to fit and no qualifications needed.
Make sure double check valve end is on the cold water supply end, drop pressure off the system and you can check for any back flow of heating water by putting the old filling loop end over a bucket.
Filling loops...