Hi Rugbyboy. Can I presume you mean the plasterboard onto the rafters? How thick are the rafters? 100mm? 150mm? Does your roof have old style felt or a breathable membrane? If it doesn't have breathable membrane, you could have the slates/tiles removed and have the felt replaced with a breathable membrane before replacing them again. You can then fill the entire space with Rockwool, Moy Metac, Knauf Rafter Roll, etc. If you use rigid insulation, you have to leave an air gap above the insulation. If you have felt, you're supposed to have a 50mm gap. You'd probably be able to leave a smaller gap with a breathable membrane. It'd be a fairly big job and would probably be expensive too.
Alternatively, you could take the plasterboard ceiling down and, if you have good head height, you could replace it with insulated plasterboard. And you should use the thickest insulated plasterboard you can. If you're doing this, it would be very worthwhile to put some insulation between the rafters also.
To get the best fit between the rafters with rigid insulation, you should use something like Rafterloc which is concertina like. It's very difficult to cut rigid board for an exact fit between rafters without leaving gaps that will lead to cold bridging.