Could anyone advise me on the safe angle a ladder should be at for
working on the outside of the house?

Whatever you do, don't climb up such a long ladder without someone
stabilising at the bottom, or one of those stabilising contraptions. Falling
off ladders is one of the most common forms of accidental death.

All the (metal) ladders I've used have had a line marked on a side panel
that should be made vertical for optimum angle.

Christian.