The brief
The Golf had been overheating, the temperature gauge inconsistent, the owner topping up coolant often, and the heater blowing cold instead of warm, with a temperature warning light coming up. He brought it in before it damaged the engine. That set of symptoms points at a worn, leaking thermostat. The thermostat regulates how much coolant flows to the radiator, opening and closing to hold the engine at the right temperature. When it wears, it can't regulate cleanly, so the temperature swings around, and on these the thermostat sits in a plastic housing that cracks and weeps coolant, which is the frequent top-ups, and a low coolant level means the heater core can't get warm coolant, which is the cold air from the heater. Run it long enough and the engine overheats, which can warp a head.
The diagnosis
A pressure check confirmed coolant weeping externally at the thermostat housing, and a check of the thermostat itself showed it wasn't regulating cleanly at its rated temperature. So it was failing two ways at once, leaking outside and not regulating inside. That made replacement the obvious call. On this engine the thermostat comes as one piece with its plastic housing, so the whole unit gets changed with a fresh seal.
The work
Enough coolant was drained to drop the thermostat housing, the old unit removed, and a new genuine VW-spec thermostat housing fitted with a fresh seal. The system was refilled with the correct coolant, the air bled out the proper way so no pockets were left, and held under pressure to confirm the seals were dry. A road test confirmed the gauge sat steady, the heater warmed up, and there was nothing weeping.
The outcome
Gauge steady, no coolant top-ups needed, the heater blowing warm, and no warning lights. The Golf went home regulating properly again. A worn, leaking thermostat only gets worse, and the failure at the end is an overheat that can cost a head gasket, so changing the unit kept it to a tidy, planned job.