My son is looking these to wear with his splints. Are they a wide or narrow width?

Great width! I am now on my third pair in a row, I keep getting them because of their comfort. The good width seems unusual in other trainers, which I have found too tight. They are also great for hip-hop!

Are these suitable for vegans... completely free of leather and suede?

I think it depends what colour you get. I think the totally black ones are not suede/leather and are OK for Vegans..

Why are the size 12UK £15 pound more expensive??

Yes, also 11UK are more expensive than, say, 10.5 – will this be fixed too? Thanks

I fancy using them with flat pedals on a road bike. I'm told they're very grippy which would work well with those pedals. Are they?

I like them for cycling, I ride trails, gravel, bridle paths etc and then my commute to and from work and they are great, really stick to my RockBros flat pedals. Im gonna get a second pair to wear out as they are a bit of a pain to keep cleaning to wear out as well.