You could hang a length of cable or push a metal rod down the chimney
from the top and then use a cable/metal detector device to locate it
from
the outside or inside walls.

I'd have to be spiderman to attempt this. The house is 35 ft tall and
accessing the chimney involves scaffolding - lots of it.

And I thought this would be a fairly simple job - I can't believe it's
not as simple as drilling a few holes in the wall!!!!

Ok, if you can't get onto the roof, then just go into your loft, get to
where the
chimney stack comes down (probably branches off into two flues) drill a
hole
into that and feed in a length of electrical cablle. put it in a few
feet and then test
below the entry hole to see if you can pick it up with a detector, then
keep going
down with it, testing every now and then to keep track of it.