Hi Malcolm, I'm sorry to hear that you have issues with your traction battery.
Replacing bad cells, 3 times in 12 months is unusual and shows that companies which take the time to balance the pack are worth the xtra £'s.
I believe you can purchase new replacement cells but for the cost, I would hand it over to a specialist and take a warranty on their 'new' pack.
Will be interesting to hear other opinions and experiences here.
Have you approached any specialists locally?
Good luck, Lee