What the French do is hack out the (weak) mortar around the stones then jam in flexible plastic conduit (having run the individual wires through conduit first) then re- mortar. The conduit therefore runs all over the place, depending on size of stone/rubble - this may not be legal in the UK.
With the bit supplied to me by TLC, I can't quite remember the exact procedure, but I do remember using a junction box and a special gland which I mounted inside the porch.
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