I think I said 'required'. Not sure it is actually part of the T&Cs of UK banks for online accounts (not checked fully with ones I use) but many of them bug you to install it.
Regarding SC and other screenshots not using SC, yes Rapport does indeed block screenshots. I have Firefox opening with four tabs, one being an https page. Rapport operates to warn against capturing sensitive information on that secure site (even when that is not the active tab) when a print screen attempt is made using the PrtScr key, but SC gives a blank SS if Grab Current Screen is pressed with no Rapport warning. So I thought if I close that https tab will SC work! SC works! So that's OK then! Sort of!
So still not a full solution, but it'll do! And I guess Rapport is protecting like it should.