Insulation update on re-roof

Looking to get my slate roof re-done. The attic has been converted into a room (with some insulation) but no felt nor insulation next to the roof itself.

So with a re-roof, the insulation is supposed to be uprated.

One quote plans to use Tri Iso 10, another says they will insulate between the rafters. The first is about 2500 second about 500.

Any advice or comment would be welcome.

Kingspan between the rafters is best. Then board and skim. If you've got room then insulated PB is even better.

thanks for the reply

the loft is already converted with a room built inside, and just a small space between the internal 'room' walls which are plasterboard with some insulation in between the wooden supporting structure. There's a crawl space between the internal wall and the room.

So the additional insulation will be on top of / in between the rafters. I'm a bit concerned about condensation in this space between the 'roof' insulation and the 'room' walls. Right now its very well ventilated ! But equally if I have to splash the cash I want to reduce heat loss as much as possible to at least feel warmer even if the savings aren't that much.