Cars are sensitive to tyre pressure, the Spridget doubly so. Garage forecourt gauges are notoriously inaccurate. Buy a quality gauge of your own and check the pressures – I run 22 front 24 rear on 165/70 tyres, but YMMV. Remember lower pressure gives more grip at lower speeds until the tyre begins to distort after which it gets vague (18 psi being a minimum). Higher pressures lighten the steering but will lessen grip. Knowing these two things you can alter the balance of your car to your own requirements. Mixing tyres from different manufacturers/types is to be avoided at all costs – one might have more grip at low speed then distort more easily so let go, another visa-versa, that way you’ll never get the handling to balance.