From what I've read, using fuel with a higher octane than your handbook suggests will make no improvement, although using a
lower octane (the 1.4tsi should only be run on 98RON) can have a significant impact on engine response and more importantly, engine wear. I've only used Shell VPower or BP ultimate since purchase, and the couple of times I've had to put in a splash of supermarket fuel when unable to get to a proper garage it's been really noticeable - the engine feels laggy and more rattly at low RPM. I've no idea what the 1.4NA engine is supposed to be run on but I'd suggest you look at the handbook, and if it says use 98 only, that's what you use from now on. A small number of tesco garages retail 99RON 'momentum' petrol, the extra 1RON won't make any difference but the price is usually slightly cheaper than Shell or BP premium fuels. The only tesco that sells it near me is a few miles out of my way, so not worth going there over the Shell and BP garages closer to my normal route.
Interestingly, if you have a diesel car and use premium diesel over supermarket fuels you find an even more pronounced improvement in the engines manners. Over time the supermarket fuel clogs up components like the EGR and leads to really lumpy running and an absence of low down torque. I pulled scoops of sticky tar and gunge out of the inlet manifold and EGR of my last diesel car before making the switch, and haven't looked back since. If you can afford it, the extra £2 per tank for premium fuel really makes a difference.