You will get a lot of opionated views on this.

Points to note: all fuel, whether branded or supermarket, by law has to conform to certain standards.

Branded fuels differ from each other and from supermarket stuff only in the additives used, i.e. the quantity and formula of the additives.

People who advocate the use of branded fuel claim its additives make the fuel burn cleaner/more efficiently/quicker in the engine and/or keep the fuel lines and pump cleaner and even that they will clean a dirty engine. Others say it's marketing hype and/or not worth the price difference.

Alternatively, you can buy your own additive, like Miller's, which claims to raise the octane/cetane levels in the fuel and provide useful extra detergent properties.

There is very little conclusive evidence about the topic, but that doesn't stop people claiming all sorts of things.

My advice to you is to decide whether you want to pay a bit extra to be on the safe side, just in case the branded stuff is fractionally better for your engine..

Whatever you go for - higher grade fuels or additives (or nothing), what you need to find out first is whether you even NEED anything like that:

• If your car is under 5 years old and does a decent amount (percentage wise) of longer distance driving whereby the engine gets fully warmed up, then it won't need so much treatment, if any, and when it does, it'll do so at an older age. This is of praticular importance for diesel car owners.
• If you do predominantly urban short trips from cold (less than 5 mins to the shops, school etc) and especially if its a DPF-equipped diesel engined car, then the engine may well need extra TLC and earlier in life to mitigate issues than can arise from such driving, especially associated with the DPF. Some cars seem to cope better with this sort of driving than others.
• A poorly-maintained car will always need more TLC than a well-maintaned one. Again, the annual mileage and service type, quality and intervals make a BIG difference, as does the quality of the components/fluids used. Scrimping on quality rarely works out in the long term.

Best to:

• Do your homework on the car you'll be getting, looking at the main and user reviews on this site (including the 'Good & Bad' section in the main review) and on other motoring sites, including (if there is one) owners clubs and forums.
• Always take people's reviews of additives and fuels with a healthy pinch of salt - some people may be acting as a shill for them, others (often from rival brands or egotistical idiots who didn't use the product correctly in a botched DIY attempt) the opposite - not helped if the person's review doesn't give much detail - such as the condition, make, model and age of the car, etc as described about - this can make a HUGE difference as to what (if any) improvements such products make.

Your car obviously isn't 'brand new' as its a 1.2TSi, which has been superseded by the VAG 1.0 TSi, so you won't have a full usage history (though may have a service history, and you can look up its MOT history [if its old enough] and what each year's mileage was).

If its done low mileage and you cannot guarantee that that was made up of longer trips, then my suggestion would be to go for a relatively inexpensive option - not higher octane fuel (or additives that mimic this) with the extra detergents, but a fuel injector cleaner such as Redex (4 tankfull bottle for £5 - £7) or similar, which is far cheaper than buying 4 tankfulls of V-Power etc over regular unleaded (£5+ per tank more). The overall review scores are a better guide than just reading individual reviews (though some may be). Halfords occasionally have products like this on good offers; other times of the year alternative outlets online have better prices.

I tested both in my 11yo Mazda3 and the Redex seemed make more of a difference in smoothness, and the superunleaded V-Power about 3% or so in mpg, certainly not enough to justify its use all year round, especially as I don't use my car for short journeys. Your car may get a larger improvement (fuel octane) as its of a newer/different design - its a DI petrol engine. This is felt more so in performance terms for the high performance engines such the 2.0T in the Cupra. The price difference (about 8-10% for branded fuels, 5% for supermarket superfuels) is, for standard cars, not worth the extra unless you go for those with the cleaning additives and you don't mostly urban short trips.

Note that DI engines suffer from more problems with carbon build up on the intake valves as, unlike with 'traditional' fuel injected engines, the fuel does 'wash over' the backs of the valves, cleaning them and is directly injected in the combustion chamber. An injector cleaner won't make as much difference on that score on your type of engine as with mine for that reason. Both will help to clean dirty/clogged injectors (likely to be for cars used for short trips and older ones).

If you're not going to be doing a lot of short journeys from cold, then its probably best using a bottle of injector cleaner like Redex or similar once every couple of years (that's what I'm now doing). Its only worth using the superfuels with the extra cleaning additives if you do lots of local driving at low speed and you find the car doesn't drive so smoothly, even after a decent service and/or use of an injector cleaner.

The 'branded' fuel injector cleaners won't harm your car and are a small outlay that you could afford once in a while. No substitue for decent quality maintenance and servicing though.