I have two sections of south-facing roof space; 

- 4m x 2.5m

- 3.5m x 2m

 

I'm planning on fitting in-roof PV to save on slates. Ideally I'd like to get around ~ 6kw but appreciate I don't have the roof space for that.

What do you think a more realistic target for my space should be?

At a squeeze I guess I could manage approx 9 panels (5 on the larger roof space, 4 on the smaller) which would be a peak of approx 3.4kw?

Use the free SMA Sunny Design tool or Solaredge Designer to mock up a system and see what fits and what you'll get.

 

https://www.solaredge.com/uk/products/installer-tools/designer#/

https://www.sma.de/en/products/apps-software/sunny-design.html

FWIW
Don't discount west facing - I've only 4kW on a west facing roof but its perfect, at least in the summer, to offset afternoon/evening use - when we start pulling significant juice. The batteries tide the system over until around mid-morning/lunchtime when theres more solar than the house draws.

Short days in winter won't help - but right now I'd go west facing over south.
Battery is full and I'm generating 3kW right now at 17:44.

I did consider it, although our west facing roof space is part of the original building (1850s) and it would be a pig to fit PV to. Perhaps when we re-roof it, hopefully not for another 10 years(!), we can reconsider in-roof PV then. 

We have the gse in roof system which works well but the bottom flashing is ribbed aluminium and does not stick brilliantly to the slates on the main house.

On the garage the installer fitted the lower panels and trays to discharge directly into the gutter, not as pleasing on the eye as fully slate framed panels but on my garage it didn't matter. Less cuts on slate and more saving.