With octopus last two years have ended up with £900 credit so hopefully my should lvl it's self out was with sse before an they would pay you're credit back every couple if months glad I switched
The membrane is only used to stop type 1 and and sub grade mixing is totally optional will not stop weeds they grow in the joints an you won't stop them
I've worked on a few jobs with these company's and must admit they are good haven't seen them on GRP roofs but large rubberbond or silerfel roofs
So would say worth a try