Thanks, all, for your helpful comments and suggestions.
Richard: As you rightly say, it's bizarre that the HW Coated paper is supplied with the printer but isn't recommended for calibration; though it's listed as 'compatible' for that purpose in the manual under the colour calibration section and the printer hasn't objected to its use on previous head replacements/calibrations. I wonder if the compatibility changed with a later firmware update?
Bill: Thanks for your endorsement of Epson Enhanced Matte, as I usually have a roll of that here for occasional cheap 'throwaway' prints and I don't really want to have to buy additional Canon media solely for calibration purposes, especially if the cheaper variants such as the HW Coated are no longer available or suitable. Just to be quite clear, I'm talking here about the calibration process for returning the printer to a known baseline, rather than the post-installation mechanical registration and alignment of the heads.
I'm sure I've read somewhere that the calibration process actually reads density rather than colour, though I can't now locate the reference. Presumably then, the colour response of the paper used for calibration is not critical, though I assume differences in dmax could affect the calibration to a greater degree in that case? I wasn't looking forward to having to redo all the profiles if I changed the calibration media, but Richard's experience suggests that is less of an issue than I'd feared.
The intention was always to stick to a known single paper for calibration from the start to avoid introducing additional variables, but Canon UK seems to have made that impossible. Apart from the Epson Enhanced Matte, I also have Epson Photo Grade Semi-Gloss Paper and Premium Semi-Gloss Photo Paper (250) available, as well as a smaller quantity of their Premium Glossy Photo Paper (250), if any of those are likely to be better options. The latter two are RC papers, I believe. Any further thoughts as to which might be the most suitable - it sounds as if the concensus may be leaning towards the Premium SG Photo (250), though I wonder if the Enh. Matte might be a closer match to the original Canon HW Coated?
Many thanks again.