When should you go back to complain

[dmat2391]

Short story, last tune up asked the mechanic shop , that we've used since the car had 8klm on it, please replace the thermostat seems to be stuck open. Opened the bonnet to check fluids because I have a big trip coming up and notice the coolant is low. It's never been low.
Looking around I see a leak at the thermostat housing. Open it up to find the thermostat has been changed but the gasket is resting on top of the thermostat (not around it) and is split.
I've lost patience with them, as it's not the first issue in the last year. I will be looking elsewhere.
Do I let them know that someone there gives no f's about the work they do?

They stuffed up the replacement clutch but fixed it under "warranty". The head mechanic seems to want to do the right thing because he made it clear that it hadn't been installed correctly.
I'm not sure it's worth the headache.
Any advice would be appreciated.
It's a limited edition manual fg ii turbo so kind of special to me