Dormer Valley type / construction detail

[Capable Noob]

Hi all,  

I've used GRP valleys where three pitched roofs fall onto a section of flat roof and thought I'd use the same GRP valleys on the other side where the dormer pitch meets the main pitched roof but I'm struggling to find any decent info on how best to do it.  The GRP valleys I have are the under-batten ones with the raised central fin and I'm using 30mm concrete pantiles (Marley Modern).  The GRP valleys aren't exactly flexible so I'm not sure how I "poke" these out between tiles while remaining flat enough to place the tile over - having screwed it in place it has to bend in order to get it to go over the tile and it doesn't seem to like it too much so I wondered about ditching the GRP and doing a hidden one and mitre cutting the tiles to meet.  Or is there a better solution?

Below pics show my use of the valley around the flat roof and also a pic I found of a dormer using the same GRP valley that I've got - any advice would be most welcome. 

Thanks all.

[nod]

Lead is much more suited to valleys 

Or even the lead substitute 

[Capable Noob]

Thanks.  Funnily enough I've been looking at the lead substitute stuff.  I may pop over to the roofing place this afternoon and have a look at it.  Does it just lay onto the valley boards and then presumably membrane, tile battens etc over the top?