One droplet of oil..

Taking the lead from a great post : http://blog.marketpoint.com/2013/01/23/hp-color-laserjet-5500-51-2-error-on-the-display/ , and with nothing to loose, as I had already disassembled the printer down to the scanner, I decided to try a wee drop of oil... (The motor that spins up the laser scanner, had almost seized up and was screaming as the printer initialized). A light clean and oil and it was fixed :)

Here is the offending scanner. There was no documentation or labelling as to which was the Cyan scanner, but the the lowest toner in the set was Cyan, so an educated guess turned out to be correct   :)
In addition, to double check that error 52.1C was in fact the scanner (and not the DC controller as the manual suggested it could be), I simply swapped the two leads to the DC controller, and sure enough the error changed to 52.1Y - the yellow scanner was the next level up.
(I didn't need to put any covers back on to run this test, just the fuser needed reinstalling, and the front door reed switch needed to be held in.)

The customer was, needles to say rather pleased at this outcome - especially as I had some years ago ordered this very part for the same error, but the issue resolved itself so we didn't actually need to change the part till now, nearly four years later!.
And when it came to replacing it, somehow I didn't have the correct part :( Luckily a drop of oil and some perseverance was all that was required.

So now I have for sale a brand new laser scanner part number C2037-69008 as pictured: