When you alter / extend / add circuits on an old rewireable fuse board how would you guys add rcd protection, assuming customer doesn't want new board ( too expensive apparently!!! )

i advise on a new board. if not then i will divert the circuit your adding on to though a seperate RCD (40A - depending on circuit) in a small housing just big enough to hold the RCD and change 3036 to a plug in 60898 in the old unit.

there is always the option of RCD fused spurs, or RCD sockets (where applicable and the cable suppying them dont need RCD protection (i.e suface, burried below 50mm)

Just for piece of mind I would test the circuit as it stands first before fitting any RCD