Chippy, the spec 30 years ago was the kind of good practice details you'd find in Seeley.
5" x 2" rafters were pretty minimal even then, depending on the span and the type of truss involved.
It is possible that adding cross bracing will stiffen the roof and prevent further deterioration, but I suspect the gradual deterioration of the structure may have as much to do with poor strutting and bracing as the kind of loading tiles might put on it.
If there is rot in the roof, or the wallplate, this will further cause problems.
You may find that the renovation grant doesn't cover major structural and roof repair, so well done for discovering it, but look at the fine print.
FWIW
ONQ.