Good evening all, I recently purchased a 153k mile 2009 S5 4.2 V8 with pretty much full history, had a new timing chain 2 years ago, freshly serviced this year. The person who owned it before me had it 2 years and didn’t use the car much, mainly at weekends. I am now using the car daily. 
Car was working fine for 3 weeks then a few Sundays ago it blew the coolant expansion tank lid off as I was parking it up. Since then it has been into a local garage who diagnosed a air lock and bled system and then it developed a small coolant leak (seemed to be coming from the front driver side area underneath), this was diagnosed as a crack in the expansion tank and that was replaced yesterday. However the car is still leaking coolant. This only happened when the car had been used for quite awhile and gets hot then I park it, turn the engine off and come back 30 mins later to a small puddle of coolant (about 3/4 litre). 
I have checked the oil cap for creaminess, smelt the coolant for exhaust gases and there is nothing. Car runs spot on and doesn’t over heat. 
I believe there must either be a hose with a dodgy connection on or I am told these also suffered from a dodgy cross over hose that goes round the back of the engine that feeds back into the coolant tank.

Any help would be most appreciated

Welcome Dominic,

Leaks, where there is no apparent outward signs of coolant, can be difficult to source. 
Yours should be easy, since you have evidence of external coolant leaking. 
Apologies for stating the obvious, but you now need to get the car to a garage where they will get the car on a lift, remove the undertrays and locate the source of the leak for you. 
Perhaps you could let us know how you get on.

Kind regards,

Gareth. 

Thanks, it’s at my friends garage today and currently having the under trays taken off 

So been on a ramp with the under trays off and it seems that the coolant expansion tank it getting too much pressure and is then pushing the over flow out. Only does this when the car is at temperature and being run for a good 20 mins or so. The garage shut at 12 so can’t do too much more today but believe it could be the thermostat or water pump that are failing causing pressure to build up. They don’t believe it’s head gasket as none of the tell tale signs are there 

Any further ideas would be appreciated 

You may still have an air lock.

Bleeding these seems near impossible, anyone got the best way to do it

Just had a proper look myself and the bottom hose coming off the coolant expansion tank doesn’t appear to be having much pressure if coolant go through it. You can squeeze it with your hand. Seems very strange. Any ideas?