Thermostat bolts shear on Citroen ZX?

I have almost no heat in my 1992 Citroen ZX, and the temp gauge barely rises off zero, even after a long drive, so I expect the thermostat is bu66ered. I asked a mechanic, and he furrowed his brow and muttered darkly about the bolts always shearing when you tried to take them off, etc.

Any truth in this? Anybody done it? Easy job?

Thanks!