When you say add a TMG, I presume that you did not have one and wanted to have it added to your licence by the CAA.
TMG Test and SRG1119 will add the TMG. Was the SEP valid at that point? Did you have enough hours to revalidate it? If so the examiner could sign the revalidation on the basis of 12 hours experience and the test on a TMG, the examiner does not need to be SEP qualified as the TMG test would be in lieu of the flight with an instructor and could be a test on any class or type!
Technically, you cannot use mixed criteria for revalidation untill you have both class ratings in Part XII of your licence therefore an initial issue TMG class rating skill test should not be used for SEP revalidation; is that what they have done? Who signed the Certificate of Revalidation? If the TMG is new it would have to go to Gatwick for issue.