The cleanest and possibly most effective way is to measure the joists and then infil using Celotex / Kingspan of the appropriate thickness. Allow 5mm "clearance" for plaster highspots through the lathes.
Rigid foam boards have superior U values. They cost a bit more but are easy to use...
Can someone clarify the law as regards installing a gas combi boiler without payment. The law seems to permit a competentperson to make such an installation in their own home or elsewhere provided payment is not involved. I should make clear that, in years gone by, I have already installed...