Hi DazzaMazza,
I have yet to convert my own garage [amazing but there you go].
It would be useful to confirm that the attic is vented properly, because this can be overlooked with small roofs.
Unless the battens are treated timber, and given that you need whatever width you can retain, I would be tempted to use one of the high density warm-board panels from Kingspan or another suitable supplier, mechanically fixed directly to the walls - no cavity - and you can fix similar boards to the rafters.
If you want to try and meet current and/or future regulations, here is a link to an overview:
ONQ.