It was only one end of the cable that was left un-terminated and open to the elements. In fact I was surprised at how little corrosion there was on the armour, at 30 cm in I couldn't detect any zinc oxide on the armour. I guess my best option is to cut the cable just before it disappears under the patio and hope that the segment under the patio and extension slab is ok - the rest of the run through the garden is easy to replace as it hasn't even been buried yet!
Thanks for your advice, Pete.