Graham, your best bet is to take an image and plate solve it with something like astrometry.net which does not need an estimate of focal length, or UniMap for which a ball park figure will get it started.
That should either directly give you the arcsec/pixel or the FOV from which you can derive the ASP since you know the pixel size.
Then the true focal length is given by:
focal_length = 206.265 * pixel_size / arcsec_per_pixel
where:
focal_length in mm
pixel_size in um
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