Problem similar to yours, started at 100ml progressed t0 400ml for 2 hours driving. Mechanic diagnosed water pump, but would only do the whole water pump, thermo housing and all pipes for 1100 pounds (150 parts + 950 labour). Did water pump myself, unfortunately stripped one thread, Still leaking, but only about 200 ml every 4 hours driving. Changing the water pump, hard, engine has to be tilted, very cramped , speacial tools. Changing thermo housing challenging. The pipe in between water pump and thermo housing, good luck. Which is what I will be doing in about 6 months. Now I know why he wanted 950 labour.