When the system successfully re-fires after a regular pause, everything goes perfectly. I have cleaned the flame sensor with some solvent & a scourer. The valve appears to function correctly with a clean action & immediate shut-off when no flame appears or power is cut. The gas control valve solenoid tested at about approx. (admittedly the supply (main) fan hasn't had any maintenance for years.) The control board confirms this with a solid green FAN light. The air flow from the PVC exhaust flue is strong. fan & the green PRES light confirms this during the start logic steps. The control board is seeing the vacuum signal from the comb. The combustion fan is working properly (I rebuilt this fan motor a couple of years back). The control board confirms this with a solid green STAT light. The RT is sending the correct call signal when the room temp drops. The will trigger another attempt (logic goes back to step 1).sometimes it will fire-up for a few more cycles, sometimes the reset does not help.

The alarm can be reset at the room thermostat (RT) or it can be reset on the control board (CB). Both the room thermostat (Honeywell Chronotherm III a.k.a 'Vulcan') and the control board on the heater go into a rest/alarm mode. So the issue is an intermittent failure to ignite at the beginning of the cycle. The OP's symptoms are very similar to mine - so I'm wondering where this ended up? It currently has an intermittent problem that is proving quite difficult to accurately troubleshoot. I have a mid 90's Brivis HE5 Wombat ducted LPG Heater. Came across this thread in my 's not that old so I thought I'd dig it up.

Brivis ducted heating user manual