It's an OMVL Dream reducer and OMVL Superlight injectors but judging by the switch the LPG ECU won't be AEB/OMVL, it will likely be something like an early(ish) AC Stag ECU.
If the switch beeps (as though you've run out of gas) it's trying to switch to gas but detecting low gas pressure so switching back to petrol as though you've run out of gas... Usually a solenoid problem.
If there are no clicks from solenoids and no beeps from the switch it won't be trying to switch to LPG, begging the question why it isn't trying to switch. The usual reason is a low temperature reading from the reducer (either the reducer really isn't getting warmed by flow of hot water or the reducer temp sensor is broken), lack of RPM signal, or if the manifold pressure sensor reads no vacuum (some system's won't switch if it seems the engine is running full load with no manifold vacuum... although Stag usually will switch even at full engine load with no manifold vacuum).
Temp sensors are a common failure on Dream reducers, especially if wires to them are tightly tie-wrapped to hoses connected to the reducer (moving the hoses pulls on the wires to the temp sensor)...If the reducer is getting hot to touch try checking the resistance across the temp sensor, should be a couple of K Ohms (it's an NTC sensor so open circuit reads extremely cold, short circuit extremely hot). They're usually corroded in on Dream reducers (dissimilar metals the temp sensor is brass), to replace it's best to cut the wires off and use a hex socket (usually 12mm). The temp sensor is shallow and easily rounded off so if the hex socket opens up at the end it's best to grind the socket so the socket's hex gets full purchase on the hex of the sensor.
Stag ECUs had the option of detecting RPM from injection pulses (instead of from a coil or rpm feed) earlier than AEB ECUs, still early Stag ECU's needed an rpm connection.
Some systems refuse to switch to LPG if they read no manifold vacuum (to prevent switching fuels during full engine load), though Stag don't usually do this. Still, since it's easy to check the vacuum connection from manifold to vacuum sensor it's a check worth doing early on. Don't forget that the map sensor vac line is usually T'd to the reducer, if the vac pipe is broken/disconnected at any point it could see the map sensor reading no vacuum.
