coolant leak

[vwnige]

changed the CTS and thermostat this afternoon..

topped up the coolant and went for a drive, when i got back topped up the coolant again thinking that would be it...

Went out for a longer drive an hour later and on the way back the coolant light came on, looked under the bonnet and the expansion tank was empty..

Couldnt see as it was dark but the only place i think i think the leak could be is the stat..

When i tightened the 2 bolts on the plastic pipe,i was been careful to not overtighten.

Could it be they are not tight enough ?

Thanks to anybody who can help.