Glow worm 30ci - shutdown at 18C - 0.5 bar error code, and leak on top up?

The system ran at 20C degrees for a long time, with power down over night for about 7 hours on clock.
Went away for a few days, turned temp to 18C - system shut down and all seemed OK.
NO!
The system has now reported 0.5bar, and a VERY gentle turn on the blue pressurization knob now results in a leak from under the left hand side of the boiler, what is this please?
If heating turned off on boiler & system power failed, we can get hot water, but no heating.
Last time similar happened we had a new diaphragm and some expansion vessel in the loft added, so why does this boiler not cope with normal adjustment of the hall temp dial, and how often should the system be bled and re-pressurized? Or have we got a rubbish boiler or install?
Thanks, Martin