We ended up using the Magma clamps but .. The cheap rail clamps we bought from Whitworths
are quite adequate for holding the panels in place but they don't have the strength to stop the panels from moving. We have two panels mounted on twin rails over the bimini so eight clamps. Four of these are the cheapies, the other four Magma. in the light of day it is apparent that we could have gone with six of the cheapies and just two of the Magma as a final clampdown. Thus far it all seems to work well. If we want to change angle of the panels we can loosen the Magmas as required.
Ultimately the size of panels will determine how much 'clamp' you need. We have two by 80w.
That said our problem now is that the our bimini frame mounts onto the pushpit, it is not anchored to the deck. With the panels up top it now has a bit of sideways movement. I seriously doubt it will fail, but the 'wobble' is a bit off putting.Somehow I am going to need to add a couple more uprights anchored down to the deck or perhaps simply a couple of horizontal angled bars.
We still need to tidy up the wiring and have the new canvas refitted but I'll post some pics when complete.