Porno confuses things because it's supposed to be ten years later, but feels a lot more like it wants to be a sequel to the film set in the present day (2002) so as not to throw off people who hadn't read the first book.
If it's really meant to be set in 1996 it seems to go out of its way not to reference anything from the time, and I seem to remember it talks about the internet and technology in a way that feels a lot more early-00s than late 90's.
Then he went and wrote Skagboys which is a prequel to the first book explicitly set in the 80's, that's full of mid-00's cultural references and commentary. He recently wrote a solo book about Begbie turning into an artist, I've no idea when he set that one.
His problem is if he wants to do anything with those characters in the present day they'd have to be getting on for 60 years old, so unless he wants to revisit them approaching old age he's stuck doing period pieces whether he wants to or not.