Its a little early to be blaming Hyundai for anything.

November and December aren't the height of summer, so it is difficult to know if aircon is becoming ineffectual. In colder weather the compressor will be disengaged anyway.

10% drop means very little as you have no idea if it was full when the car was new.

I would leave it until spring/summer. Then if there is a problem it can go in for a proper investigation which usually means refilling and using a trace dye to identify any leaks.

All the same you wouldn't expect it to need a re-gas after 15 months.

You can get this done for between £40 and £50 at places like ATS, or find an independent garage nearby with the right kit.

I'm surprised the Hyundai dealer didn't just do it as a goodwill gesture, but more worrying, I wouldn't expect a 10% loss of gas to affect the aircon's performance that much.

Any place which re-gases for you should also perform a leak test, so if you find it's the aircon compressor or the radiator which are faulty, these items would be covered under the Hyundai warranty.

I had my 2007 Mercedes A Class re-gassed by ATS last April after it stopped blowing cold, payed £39.99 I think. After a few days it was blowing ambient again. Took it to Merc dealer (who wanted £150 for a re-gas!) and they found a tiny hole in the compressor/radiator. Replaced and re-gassed free of charge under the Mercedes used car warranty.

So don't give up with Hyundai. As said above, no manufacturer would cover a re-gas as a warranty item, but try and find out why it has stopped blowing cold in such a short time.

You say you bought the car in September 2015 - was it new? If so then it should still be within the six months warranty coverage that Hyundai provide for air conditioning gas.

No, OP states it is now 15 months old.

The Hyundai warranty specifically excludes -

"The replacement of lubricants, fluids or refrigerant other than as a direct consequence of a warrantable repair."