I've managed to get all of the eaves and rafters to play nice with each other. The plane of the battens either meets the edge of the brickwork, or steps in a little. This would work OK for the eaves and first course of plain tiles. A bit late in the game I thought to try and squeeze in an over fascia vent. The vent itself is 25mm tall and if it's fixed to a 19mm batten it creates quite a kick up.  I haven't come up with a way to secure the vent to the brickwork without the timber. The gap in the image caused by the kick up looks huge - it's probably 12-15mm.

 

How much kick up is too much?

 

 

Typo -> The vent itself is 15mm tall. 🙁

I think you need to put some tiles on there to see how they lay.

Think about your dry verge system as well.

The problem we had with a large kick up at bottom of roof was with the dry verge system  on the gable end, that we had planned to use on our slate roof.

We had planned to use a continuous dry verge, an aluminium pressed strip in 3M lengths , but it just would not fit because of the kick. Needed a straight edge.

I've put a few rows on at the gable. Vent and batten will kickup ~34mm. It doesn't look wrong with a 38mm kickup from the brickwork.

 

I hadn't considered that it might upset the verge. It looks like the verge I am planning to use will accommodate.