Comsumer Unit protected by 500mA RCD

[Newcomer]

I'm recently qualified Part P which as we all know does not make you an electrician so I'm checking out as many installations as possible to widen my experience of the problems I'm like to face. I recently came across an old 4-way wire fuse board protected by an separate old 500mA trip/isolator in an late 1970's flat in a block in Folkestone. If I was to replace the board with a new split consumer unit with one half protected by a 30mA RCD, would I also have to replace old the trip/isolator? I've yet to confirm the supply system with the supplier but the supplier fuse unit would suggest it's a TN-C-S supply. The flat is owned by an elderly lady and so I want to keep costs down for her and new time delayed RCD's are a little pricey.

[Jimbo]

What use is a 500ma trip going to be? it will not help in the slightest, install split load board and protect all sockets for use outside the equipotential zone with the 30ma RCD. Job done. Regards Jim.

[Newcomer]

No use at all but using it switched off as an isolator while I change the board makes life a lot simpler. I'd planned to use a split board with a 30mA RCD anyway but I just wondered if there would be an conflict if I left the old RCD in place. What do you think?