Well, I’m glad I stopped being “tight” and bought the injector.
Rim tape on nice and tight, tyre on, pump and “pop” with the valve core out, inject jizz, pump up to 35psi and… they seem done!
The onle bit that made me nervous was that as I tightened the valve in it started pulling the tape away from the edge. It stopped with some overlapping the rim still which i ensure was “stuck” with some thumb rubbing. I guess the rubber under valve will put extra pressure ‘twix tape & rim to seal it tight anyway, so here’s hoping I don’t have a failure at the weekend!
I had so much extra time in my lunch hour I managed to invisitape my seat stays!