Hi there

to whoever can help me (please)

I have an Audi A4 2.0 tdi estate I had issues with the coolant leaking and now I have replaced both thermostats and the expansion tank it’s stopped leaking onto the floor but the coolant keeps dropping does anyone have any any experience or advise 

my car has 151k miles on it 

 any thanks 

Hello whoever you are -? 
Can you please take a little time to complete your profile - thanks. 
Anyway, 

Did you renew the cap with your replacement expansion tank? Can we take it the replacement tank was new rather than secondhand?

Did you thoroughly bleed the system after replacing the above?

How much coolant is being used in say 100 miles?
In addition to answering the above, I would suggest you take the car on a run of at least a few miles, and immediately get an assistant to rev the car to 2,000 rpm and hold it steady at those revs. for 2 minutes, while you observe the exhaust for any evidence of white smoke - or undue smoke. 
Perhaps you would come back to us with some answers.

Kind regards,

Gareth.