Thanks for all your replies!
Studs are a plain matt black - no coverings.
On closer inspection (without having to take the bag apart!

) it looks like the studs are screwed on rather than pushed in. Which means my lost stud must have been loose for some time before falling off!
I wouldn't have been so upset if I used the bag everyday 'cos I'd put it down to wear and tear... BUT it's only been out of its dustbag a handful of times
Anyway, I'm gonna stop being a baby and give the store a call in the morning, see where I stand with this.
Should be able to get it sorted out for free, seeing as it's a case of getting a spare stud and screwing it back on!
I'll be back with updates!
xx