Quick update!
I deciced to go with the adding more barbs, this time smaller green barbs, as recommended by by local shop, to the tank to see if they would just fight amongst each other and leave the shark alone. This worked for about an hour or so, when to my horror the tiger barbs started on every thing in the tank, and I do mean everything!
So I the spent the next hour trying to net the tiger barbs, and they went back to the shop, who very kindly swapped them for some nice safe, sensible neon tetras!!
Lesson learnt!
Thanks for all the advice and comments!