Thanks very much to all who replied to my question.

Basil, thanks for your advice. You wrote:

If the sloping bits are only short in length and the above isnt worth
the effort I'd just put insulated plasterboard on the inside
attatching with nails or screws.

The sloping bits are only about 300mm long. I really don't want to replace
the plasterboard - I have limited diy skills! So if I leave the existing
plasterboard / lathe and plaster there, are you suggesting that I add extra
plasterboard to the cold (loft) side?

I had wondered about putting polystyrene on the cold side, but still leave
some gap to the eaves. Any thoughts?

Thanks again

David