The electricians have used what looks like a 16mm² cable (7x2.5 strands) as the main ground connection from our 3 phase load centre. I'm told (but haven't yet checked) that there are three 2m rods buried outside the kitchen. Is that a reasonable arrangement and shouldn't the cable be sheathed where it leaves the house?

Looks kind of dodgy to me...

I would have expected the insulation to carry on all the way to the rod, but it's not hazardous like that, just untidy, once it's filled and painted you'll never notice.

I am concerned that the connection to the rod(s) is not accessible, unless it's been welded (as PEA recommend) you need access to the clamp for maintenance. Time to start digging