I tried this again with my own details. All seems fairly straightforward until the page that asks if any endorsements have been made to your licence since it was last issued by the CAA. It's unclear if that includes the latest SEP revalidation or not, but happily skips this if you don't say anything and comes up with a calculated fee of zero.
If you do choose to add a new endorsement, it then asks for a certified copy of your ratings pages to be uploaded. However I understood that an instructor with FCL.945 can't certify documents such as ID or licences so you'd need to seek out an examiner.
I wasn't presented with any option to upload other SRG forms mentioned earlier in the thread. Perhaps they have recently changed the system, or maybe it was because I ticked "Professional Licence".
After proceeding to the next stage without uploading said document, the system went into a continuous loop swapping between alternate webpages
https://ids.caaexams.co.uk/issue/hrd and an SSO sign-in page and refused to display or accept any further input.
The previous attempt, I didn't try to enter anything in the endorsements page and it let me proceed to the charges section, confirming that a my application won't be processed until payment of £0 has been received.
Methinks there is a tiny bit of debugging left to do plus some clear communication wouldn't go amiss.
Nonetheless, this could be a helpful step forward especially if it improves the data quality of licence records kept by the CAA online.
UK Flight Instructor with TB20 share at Gloucester
Post PPL flight blog