Mike
The Yellowtongue is a great idea. I have recently finished lining my shed with 12mm C/D ply and did the same - attached direct to the top hats.
Tony, I reckon you are on the right track with the lining and insulating. I used Tontine sound proofing insulation and am very happy with the result. I did leave the sheets around 20mm off the floor so it could be washed. I was planning on finishing that off with lengths of 50mm Al but not sure I will bother. The ply is reasonably weatherproof as well. All the cabling was run in conduit before lining was installed. I went with the overkill option so no need to run any more later.
Geof