Make absolutely certain that you insulate any pipes in that loft well. By that I mean the black closed-cell rubbery stuff (armaflex, preferably the 25mm stuff).
Otherwise like a neighbour you may come home to water pouring out from under the bargeboard for several hours (could not contact them and could not risk turning the road stopcock off).
If your walls are very damp solid brick, suffering wind driven rain, with a wet verge roof of tiles on rotted felt on mortar, then Celotex between treated battens is often not so good. The wood will rot and anything but stainless screws will split horizontal mortar courses (got a wall to repoint from just that next year). A better solution can be bonding 40mm Marmox to the wall with Keraflex, it is only
2/3rds as good as Celotex and even more costly - but you stick it on like buttered toast, no mucking about with battens.
If your walls are reasonably dry, that is to say the roof on the lean- to is good, then Celotex between treated battens works very well. Even
40mm Celotex is eye-popping going from "impossibly cold to no heating required". I would not use rockwool on solid double brick having seen it turn into a sodden mass in 6 months from interstitial condensation whereupon its insulation value was zero and it froze the pipes in the wall through direct conduction whilst it was being stripped out last winter. Celotex (& Marmox) are closed cell foams, Marmox is aimed at shower/bath/kitchen/wet-room areas.
If you are electric heating, realise too little will just not work because you can not build up the thermal mass which takes 4-5 days. Same goes for GCH - if the radiator is undersized or not zoned to come on more often then you will not get enough thermal charging because once it cools off it has no chance of getting back up to temperature. Sometimes a bigger aluminium radiator (or even better the old cast iron) can help to get more heat output in a given (limited) space. Failing that a 700W oil radiator can help, or an on-demand electric fan heater like Dimplex FX20V/VE/VL, or an underplinth electric-fan- but-wet-radiator system.