Hopefully i can help..
How significant is this leak or is it just a straight steady stream of water as you fill the expansion tank? so i asume its leaking out the engine gearbox side?
I see you say it been pressure tested? how was that done - and what else if anything has been already inspected?
Hi..there thanks for your reply..
Doesnt sound right - you have leak in the coolant system and the pressure test didnt pick anything up.might be time to seek another opinion or test at another garage if you feel the the first test/checking wasnt done properly .you dont say say what type of pressure test..was it a coolant system test where you place a hand operated pressure tester which pumps up the system to that pressure specified on the cap this is in turns forces the leak to appear and thus be able to track it down - or did they do a compression test on the engine that would likely come back ok if the coolant isnt being lost via headgasket related...If the leak is significant to the point of just going right through the system and out onto the ground then said leak should be easily spotted..maybe if you can - try a garden hose so it continually fills the system into the expansion tank and then make checks that way so it creates steady supply of water..then you should without a doubt track it down by simple following the coolant system circuit around the engine until you find the the route cause for the coolant loss..if its from the opposite side then make checks around the thermostat and it housing to the side of the cylinder head on gearbox side or depending on engine "near the coil pack where your ht leads go" - aswell as the hoses to it..housings/seals on these are common for leaking and require total renewal inc thermostat - if your sure it not leaking out the pulley side of the engine and leaking out via the crank pulley then you can maybe rule out water pump..if its from the front of the car then inspect the radiator sides and lower area around the core/and large hose connections..front end damage to the radiator etc..also check engine freeze plugs/headgasket seem between the head and the block..both hoses that lead to the matrix..the key as you may know yourself is to follow and trace the leak which you have spotted "which means the system has definate leak"..its really a case now im afraid of tracking it down there's no other option or advice really bar what ive advised..I do hope this helps further