Now done it, and in annual at present
Creating the SDMP using template and MIP wasn't too onerous. Really a matter of understanding which boxes to tick, going through logbooks so that I could transfer AD and time-lifed items into the required annex page. Have submitted to CAMO with everything necessary (including the aircraft) and await his comments. We had discussed it before I did the final document and I believe have got it right, but everything is on a memory stick too, so if he spots something that needs tweaking, no big deal.
Saved £300 by doing it myself. Yet to see if annual cost is any less, but there are future savings for sure on certain components. CAMO was also uncertain about the need for radio annual under new rules, so I'm going to do own testing (and keep records) until someone says different.