Due to other projects, this got pushed back a little - but I have managed to have a chat with National Grid about what I want to do and all of my application specific queries.
The upshot of it is that they would be willing to up rate my current grid allowance to 7KW - so basically an additional 2KW.
Although they are happy to let me have more behind a G100 export limiting install.
So basically it looks like planning permission next to change a flat roof to a pitched roof.