Don't worry. Got the answer. I spoke to someone at yambits.co.uk and they said it's a dealer-only part. I emailed by dealer and they responded with:

"The tube comes as an assembly i.e. tube and grip at
�14.62 and the L/H grip is separate at �6.88. We do not stock these but can get them to order in 3-4 working days."

So just got to decide whether to dismantle the old bar or spend �20 and fit new grips and tube to the new bar.

Hmmm.......