Got a call from my tenant saying the "the switch in the fuse box keeps
repeatedly tripping". It was just the main downstairs ring that was
affected, the other circuits and lights were ok. Got her to unplug
all the devices and she said the problem was still there.

As I don't live that close to the property I decided to call an
electrician to have a look. He said that he was not able to identify
the problem but had bypassed the RCD to restore power temporarily
until a permanent fix could be found. He also emailed me a quote for
£600 for two man days work to identify the problem.

I decided to visit the property last night just to make sure I was
happy the tennant and the house was safe. I tested a few of the
sockets with a socket tester and it showed that none of them was
earthed. I also managed to establish that the fridge which is located
in the middle of the kitchen "island" was not receiving any power.

IIRC when I tiled the kitchen floor there is a spur to the island from
a JB under one of the tiles (don't ask - I didn't want to mess around
with the existing electrics). So maybe there could be problem with the
JB surrounded by tile cement / concrete?

A couple of questions do I have two separate problems here?
1. No power to the kitchen "island"
2. An earthed live or neutral some on the ring (or possibly even an
appliance that wasn't unplugged). I guess it's possible that there's
a spur that we don't know about that something is plugged into, or a
mouse has chewed through a wire

Are the problems are related?
Is the quote from the electrician reasonable?

Any help gratefully received