”Sorry to be a pain, but will I need a transponder to fly through other class D airspace from next year.”
In the UK you will not need one unless the area is designated a TMZ (Transponder Mandatory Zone). The Scottish TMA above a certain altitude and the London TMA for now, not aware of any others coming in.
Whilst you do not need mode s for class D transit I am sure you would get easier transit with it.
“And if anyone can tell me how I will fit an air-ground switch to a Tipsy Nipper please let me know. Are they having a laugh?”
As I said it would be very hard in some aircraft. The LAA will approve a standard installation without the switch and you do not need CAA mod approval so I would not worry about it.
I have mixed memories of my Nipper ownership (G-AXLI)
Rod1