How to top-up a Glow-worm Micron domestic boiler?

Asking this question for a distant elderly relative, before she gets ripped off by yet another crooked plumber. Her heating system has had a hidden slow leak from a thernostaic valve connection for several months - want to know if it is still safe to start boiler. Boiler only serves 4 ground-floor radiators &indirect cylinder.
1:-Does the Glow-worm Micron 50ff boiler use a pressurised sealed system (but NO pressure gauge on boiler)and need topped up by a filling loop?
2:-Or is it an open system topped up from an ever filling expansion tank, and therefore safe to restart after curing leak. Many thanks in advance.
3:- Does the Micron 50ff incorporate a Low Water Cutoff Switch / Sensor?
I cannot get any joy from Glow-worm themselves, so many thanks in advance

JohnMcKay
January 2010