Anybody know if there is a shop specialising in suitcase repairs on the island?

You could try the girls at the "Foam shop" near Maenam Resort road on the ring road ... on right side coming from Maenam Soi 4 (traffic lights).... It's usually fairly easy to spot... probably not open Sunday!

Fairly close to Samui Airport arrivals gate, on Sanambin road going towards Bangrak from the arrivals gate, on left hand side - I recently noticed a "Luggage repair" shop with sign. Fairly big shop set a bit in from the road. I believe it is called: Nattakorn Samui Baggage Repair Service Shop

Thanks. I'll try them both. Airport one first then if no luck I'll go to Maenam.