Solar install on old stone flag roofs

[gcm]

I have an old (perhaps >100+) stone flag/thick tile roof which is most likely to be kerridge sandstone as that is local - last refurbed ~80s+90s.  Rather Heavy.  
Not too concerned about incremental PV system weight but it will need care and thought in terms of what and where it is anchored to in a barn conversion roof construction.
A windy exposed site. 

I have reservations about a ballasted over ridge frame drop on and tie it down approach - solar sailing !  but I am open minded at this point about what would be the safest and least invasive approach in context.

I am interested in others' experiences with installation approaches and suppliers for PV on barn conversions / this kind of roof.  This is retrofit as a full renovation in not in progress and it was converted slightly too early for this to have been done at the time of conversion.

I am cautious about disturbing an old roof outside of a broader renovation and perhaps uncovering latent requirements - it doesn't leak at present - it would be a shame to trigger this and accelerate wholesale roof works.

The site has PV potential as there are a couple of large SE and SW aspects which could easily hold the maximum allowed FiT system or much larger 3x-5x in terms of roof space.  It is very tall for a 2 storey building and the roof (and cars and everything else) are prone to moss growth. I am also slightly concerned about maintenance access for cleaning without scaffolding having not researched these aspects to retain output yet. 

Direct experience or thoughts on things I should be thinking about in up front in planning other than carefully considering suppliers (builders, roofers, sparky) with the right credentials for this type of building.