Well the update on this is that we chickened out. Try as we might, deflating the tube all the way down one side of the boat, it is simply impossible to gather up enough 'slack' to hold onto the inner part of the valve. The fabric is too tight/stiff especially as opposite the valve is the wide rubbing strip.
If we unscrewed the valve we'd never get it back on again, so aborted!
Instead we put a thin tube through the valve and pumped the sealing fluid through that with a modified garden sprayer. Hopefully we managed to manipulate it down the tube to the right spot!
We'll see if the tube holds air in a few days when it's had a chance to cure.
Thanks all for your assistance