On Sun, 4 Oct 2020 12:33:29 +0100, F wrote:

What is odd is that with the actuator head off, the lever springs
immediately to the off position but with the head on it either doesn't
move back at all or, as just now, creeps across very slowly and not
quite fully. As though it's having to move the motor's splined spindle
as well as the valve?

Not clear to which bit the lever is attached, the acuator (normal) or
the valve body (not normal). As it's sort of trying to close my guess
would be that the valve is slightly ceased, take the head off and
fully operate the valve stub shaft both ways fully a few times and
see if that frees it up. Had a honeywell valve do that on me, once
free'd off it worked for years with no further problems.

--
Cheers
Dave.