Best way to replace Sunvic SM5203 central heating valve actuator ?

The Sunvic SM5203 valve actuator in my CH system has packed up. Sort of - it's the two-way, five wire flavour with two microswitches to limit the actuator travel instead of a spring return design, and that's where the problem comes from. The microswitch which confirms "valve closed" and stops the motor is iffy, resulting in very quick oscillation (about 5 times per second) when the thermostat calls for heat, and zero movement of the valve results. Occasionally, just to make it awkward to troubleshoot.
I've had the thing apart and the problem appears to be a combination of wear and a design fault - part of the mechanism is held on a subchassis which in tun is held by two spacers, secured with circlips on top of the spacers. That mounting has too much play, and that's what causes the microswitch to oscillate rather than open or close definitively. Would need at least three mounting points and screwing down rather than circlips to eliminate the play.
I've tried replacing the actuator before, but the original Sunvic (!) replacement came with a very noisy new motor design (loud enough to wake me up at night, dammit) and still had that same design fault.
I'm reluctant to replace the whole thing (actuator and valve body together) because that means draining down the system and doing plumbing work, which I hate. On the other hand I don't want things to stay as they are - if I'm away for a night and the thing fails again I could wind up with frozen pipes!
So, is there a way to repair this thing properly, or is there another manufacturer's actuator which will fit on the Sunvic valve body ?
Any answers and pointers appreciated.
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