matt_cambs wrote:....I obtained around 150 hours in an Archer and last flew in January 2019. The IAA licencing department referred me to ‘subpart C of Annex III to EU regulation 1178/2011’ (Easy Access for Aircrew regulations on the EASA website)....
Matt I'm not sure why the IAA pointed you towards Annex III of the Aircrew Regulation, did you get an FAA licence with Single Engine Land rating while out in the USA?
If not, Annex III is not directly relevant to your situation because it is dealing with licences / ratings from other states and the issue of licence / ratings on the basis of those.
In terms of getting an SEP on your EASA licence, AlanC and others have it right I think, although the ATO/DTO may wish to contact IAA for clarification in advance of doing the training and test, I don't know how many SEP additions to an existing licence they do. IAA licensing are generally pretty good but it is slightly unusual because most initial licence issues are PPL+SEP rating.
If you do have an FAA licence with SEP (or SEL I should say), then the Annex III route is also an option.