Mrs g road bike has a bit of a blockage I have tried to thread new gear cable in but is blocked at BB from either end there doesn’t appear to be any damage to the guide tube Apart from this frame is in good nick (2017 boardman air 9.2) anyone sorted similar? 
Blocked at white marks
You may need to remove the guides and inspect them, it takes very little wear for the indexing to deteriorate. My Giant Defy gear cables abrade through those guides every 5000 miles or so. They're cheap to replace but a pain to feed round press fit bottom brackets. And if there is any wear, it's impossible to feed the cable through.
ok thanks for that, does your defy have an alloy guide up the down tube ( ours does)if so how do you join a replacement to maintain the tube? I had considered some drilling to allow a full outer to be run in the down tube?
I think you tube may be your friend, or your LBS. My defy has the plastic guide tubes running from the headtube all the way through. Maybe the plastic guide tube on yours runs inside the alloy guide?