Just to come back on this one, I’ve not had time in the interim to try these again, so thought I’d have a go tonight. The bike has had the tyres on with tubes in it for ~7 days/~120 miles.
I didn’t touch the tape, didn’t make much effort to not disturb the second bead, just took the tubes out, popped in the vales and attached a compressor, both popped straight on within a second!
Added Stans goop through the valve and both are holding pressure.
Definitely worth having tyres fitted with tubes for a while first if people are struggling.