I was on the scrounge earlier for a used/faulty Sunvic SDM1901 CH valve
actuator, to keep as a standby for the one I installed a couple of
weeks ago. I have now sourced one.

The full explanation - My CH for many years has relied upon an SD2701
spring return type 3 port actuator, which is a very unreliable unit,
running its motor constantly against a spring. So unreliable, I
habitually kept at least one spare in stock and often repaired them.
The 2701 has been by far the most unreliable part of my CH system since
it was installed decades ago.

The 1901, although being an electrical direct swap for the 2701, is a
far, far better design, with no spring involved and rather cleverly the
motor only runs whilst adjusting the valve setting. The actual valve
also has to be swapped too, involving a drain down and refill.

One of my roundtuits, was to try to completely redesign the old 2701
actuator so as to avoid its over complicated unreliable design. No need
now - I do wish I had found the 1901 years ago.