Have had my focus 6 months now. From the start i noticed it using around 1ltr of water each 20 - 30 mile trip. Then i noticed expansion bottle was going black. Car back to indy dealer. They figured oil cooler was issue. Replaced. Coolant system chemically cleaned and flushed. Next day still losing water. Back to dealer. Turns out head gasket had blown before and previous owner fixed it using a pour in sealant. The chemical flush appeared to have removed the sealent, so now head needing done. New gasket, still losing water. Have since had replaced Timing belt, Water Pump, Thermostat and various hoses. Still losing water, and forgot to mention the temp guage swaying around. Also when pushing it a little, around 80mph on my private road the temp guage hits max for 10 or so seconds then returns to midway.

Car has just been pressure tested and head checked for gasses. All showing normal. Any ideas folks ?

Goes in tomorrow to be smoke tested for leaks.  They fiĺl the system with smoke and using filtered glasses and a uv light they hope to discover something.  Porous head was mentioned as a possibility. Fingers x'd but losing the faith

Well that got us nowhere. Back to the drawing board. Feel free to offer any suggestions, no matter how outlandish they may be !

not 100% on your engine but maybe egr cooler if it has one maybe split internally and leaking exhaust fumes  into water?and vice versa burning water through engine and exhaust and im not sure smoke uv kits works well on a diesel 

Tnx Dennis, by chance I noticed froth coming from a joint at 1 side of the EGR. Pointed this out to the mechanic, tho we could not get it to show again.

Since the car is being repaired under warranty, and he was not happy with the way the previous garage had routed wires and pipes after head gasket was done, he has decided to do the head again, as well as  cleaning the EGR and replacing gaskets for it. Should be finished next week. Fingers x'd this sorts it.