And to answer my own question, a call to YouFirst to change the time on a domestic add-on would result in a reduction of £20 per person - surcharges for the domestic leg only, plus waived change fee as I cannot do it online. So no recalculation of the whole booking, and thus no extra mileage charges.

I haven't made the change yet so this is not set in stone!

Once again, YouFirst were excellent ^