Moving home again,I've found my old Horseman 6x9cm Polaroid back which won't fit onto my Graflex Century.

It is a 2x3" film back, however there is so little space to activate the clips and hold the Polaroid holder in place. There is also an L bracket for adjusting the front spacer to correct for the space.

I've been trying to figure out how I got it to work on the Graflex Century before. Talk about amnesia.

Anyone have any experience with this?

From what you describe, that holder doesn't preserve the film plane (if it uses modern pack film, it couldn't do so and still fit a Century; the film packs are wider than the space in the 2x3 Graflok back). It sounds like you would install the back (which might be tricky, but once done should be no big deal) and then use the "bracket" to set back the front standard by the same amount as the setback of the film plane, so as to preserve function of the RF and/or focus scale.

For getting the back on, there ought to be enough room to operate the slide locks; you'd pull them back, lift out the focus panel, drop the Polaroid back into place (with the rib engaging the groove in the camera back) -- and there should still be room to slide the locks back in and lock the back into place. If there isn't, there's something else wrong...