If I were you Al, I would stick in as much as you can – pull the old ceiling down, leave an air gap under (I assume sarking board in the ‘burgh)of about 40mm, then a soft batt of rockwool, then as said above a foam insulated plasterboard or aerogel right across the whole lot.
I would not batten it out as first suggested – you end up loosing a lot of the effectiveness of your insulation by cold bridges through the timber.
Insulation is (relatively) cheap at present, especially compared to increasing energy costs.
And yes it will make a huuuuge difference.