It looks like I'd have to break the seal to remove the fuse to allow me to detach the main tails into the csu.
I know you probably should get permission to break the seal (they'd probably insist on a qualified electrician to do the job + resealing - just trying to save some money by DIY), but it appears very easy to be done.
Then in goes the new csu with rcd and I'm update with spare fuses.
Any thoughts though on this main tails/fuse seal issue would be welcomed.