Can anyone identify (what I believe is) a surface mount transistor
labelled as "43" ?

It looks like a SOT-23 package. Possibly PNP looking at the circuit.

This appears to be switching the gate voltage on a triac (switching 240V
mains). The transistor itself is driven straight from a microcontroller
with no limiting resistor ???

My searches match only a Schottky diode barrier - which seems unlikely.

Thanks,

Mark.