To instruct for a Rating the Instructor must hold a valid Rating that, as a minimum, gives the same privileges as the Rating being taught.
I have a current MPA-IR, but am I right in thinking that I need to renew the IMC in order to instruct instrument flying on a single pilot aircraft
As an MPA-IR is not valid for SPA then, at least, an IMC Rating needs to be held in order to instruct for the IMC Rating.
Pedantic Semantics: RTN11, from what you have written do you, in fact, mean that your IR(Restricted) has lapsed. Most Instrument training will be done on EASA aircraft and an IMC Rating is not valid for these.
The amount of refresher training required to reach the standard to pass the proficiency check is entirely at the discretion of the ATO
This is quite true but the question does arise as to whether this is a Renewal or is, in fact, an Initial Issue?
It can't be an initial issue if you have already held the privilege
This is not correct - Holding the privilege to do something is not the same as holding a specific Licence/Rating.
EG an FI holder includes the same privileges as those of a CRI SEP holder, but they do not hold a (stand alone) CRI Certificate - they could get one but would need to apply to the CAA for Initial Issue.
Only the CAA can add a new Rating to a Licence, Examiners can only Revalidate or Renew ones which are already present, they cannot write in new ones.
So the question is:
Is an 'IR-SP-ME class/SE' a single Rating or is it an abbreviated way of writing 'IR-SP-ME class' and 'IR-SP-SE class' and therefore an Examiner may Renew simply the 'IR-SP-SE' class on its own.