Coolant system over pressure?

Hi All
I've a 1991 2.0 190E, Petrol, Man G/B 130K with a odd coolant problem. I'd been loosing coolant and found the seal had gone in the expansion tank cap, so changed this. Then found coolant on top of the engine under the injectors -traced to a leaking hose union, which I fixed but I was still loosing coolant at a rate of 0.5 litre every 500 miles. I took the car to a mercedes specialist and had a 36K service with a coolant flush, a new water pump and thermostat fitted. The tech pressure tested the coolant system and noticed a very slow drop in pressure which we put down to drips of coolant from the water pump. But the leak persisted so I rigged a plastic bottle up to the expansion tank overflow and drove my usual 500 mile journey at 80 mph with a couple of stops. At the end I found 0.5 lites of coolant in the bottle! So for some reason it seems the system is being over pressurised. The car runs brilliantly, temp sits at about 80, the oil level is fine with a good clear colour on the dip stick and the exhaust is clear. The expansion tank is a bit gungy with a sort of brown sticky residue in places and I noticed what looked like tiny aluminium particles when I flushed the tank - some sort of leak stopper maybe?
The only other odd indication is the heater slowly cools off when travelling at speed, although this has improved since the coolant flush.
My thoughts are a head gasket leak of some sort but I'm confused by the symptoms. Maybe there is a slight blockage somewhere causing the pressure to rise in the system?
I would be grateful for your thoughts!
Cheers
Nick Dormor :-)