was told by my local Kia dealership (Epsom) that the warranty is valid as long as I have paperwork showing the car has been serviced by ANY VAT registered garage using original Kia parts. Although I don't have a stamped service book, I do have paperwork showing the previous 3 services that had been undertaken by a KIA dealership.
First of all, welcome to the forum, 'sasmjuk'.
With regard to your question, I agree totally with 'iooi's response to you but I have to question why you are even thinking about having the car maintained outside the KIA network.
As for the missing service record, why is it missing? That is most strange and if you purchased the car from a KIA dealer, I would expect there to be the official service document, all present and correct. Loose paperwork is easily forged.
The fourth service is a major one and in my opinion should be entrusted to a KIA dealer. That the vehicle was sold to you with a service due makes me suspicious. Is it due or overdue? If it is overdue, then in the event of a major disaster requiring repairs in the future, that you believe should be under warranty, you may find that KIA disagrees!
As there are shedloads of 2nd-hand Sportage models on KIA dealer forecourts, most with impeccable service history, I hope you purchased at a fantastically low price, particularly as the official service document did not accompany the car.
Would you guarantee a new vehicle for 7 years/100,000 miles then agree to meet a major warranty claim if the owner(s) had used independent garages with paperwork that probably doesn't accord exactly with KIA's rigorous service requirements? The 7-year warranty is a two-way street. The 1st party, KIA, promises to repair things laid down in the terms and conditions.....provided that the other party, the owner(s), adhere rigidly to the servicing schedule laid down in the owner's handbook that accompanied the car when it left the production line.
While there are some people who pop up in these pages condemning certain KIA dealerships, there are far more poor independents than there are poor KIA dealers. Are independents cheaper? Quite possibly but not by a great deal. KIA's labour rates around the country are generally pretty reasonable by industry standards, unlike some German marques or indeed, many other well-know brands. That is why I question your intention to entrust your car to a garage that will not even be able to update the infotainment unit or satnav maps, nor perform the bodywork check.
I really hope you chose well!