Tyre pressures in my Haynes manual are given as 17psi front and 23psi rear.
This seems rather low,and I'm wondering if this is for 'stone age'tyres.
The bike is shod with 325/19 Mitas front and rear.
Are these pressures correct?
No, they are too low. Try 25 front and rear for solo riding and experiment. Rear may be better at 28 but see how the tyres look and how it rides.