Contacted CS, they offered £1.50 extra for move to BB XL, but £7 extra for BB L which seems silly.
Put through to customer relations and was offered(didn't threaten to leave or anything like that, just asked for a bundle deal) £38.75/month(was £50) on an 'indefinite basis' moving from BB M to L. That's better than the deal for new customers. Upgrade to BB L was even done quickly whilst still on phone. Paying £11.25/month
less for an
improved service. Result.