OP; be careful and sensible and keep at it 😯
I’ve been fixing my own boiler for the last ten years.
I only began because I couldn’t get a so-called professional to turn up when he said, do the job he quoted for quickly and efficiently or leave the boiler in reasonable condition having completed the work he was being payed for 👿
In this time I have never disturbed a gas pipe joint. However I have;
Replaced a pressure sensor
Replaced the water circuit flow sensor
Replaced the flow sensor Diaphragm
Removed and repaired the Flue Fan
Cleaned the burner unit
All quickly, efficiently and without overcharging or damaging the boiler. I maintain that I have had less time without hot water and heating than I would have had if I’d relied on a ‘professional’ 8)