Well I went out for a look yesterday and a car mad neighbour (4 cars inc TVR) came out and offered his time, driveway and trolley jack and stands!
After a short amount of running we could see nothing at all but once at temp a high revving revealed leaking near the front middle. I had to get underneath and wet to find exactly where. I then noticed a completely sheared, hanging jubilee hose clip.
So for reference it was the clip on the water pump facing out toward the right. The clip stated 30 - 45 mm but I could only source 40 - 55mm. To get it on I drained the header tank using a coolant checker pump. Disconnected the top of radiator hose nearby for access drained a little more then removed that S shape hose. Then drained a little more water from the hose.
Without removing the hose at the pump I opened the jubilee fully and then swore MANY times trying to get it back together while over the hose with the other chap standing above and me with my arm below the car. I probably could have removed more but with recent luck didn't want to chance something breaking - I was very happy to find a hose clip and as yet nothing more sinister.
The clip was wider so didn't 'sit' right into previous clip groove, I really hope it will be ok as it was a pain making those shapes. If I consider a timing change I may get them another clip and renew coolant fully.
Thanks for the kick up the butt E_T_V ..."Coolant loss isn't normal - so find out where it is going!"
Now waiting on intercooler hose arriving, not sure I'll fit myself ... but it looks a whole lot easier than this was?!