Had waterpump replaced and belts, but the reservoir keeps running out of water in traffic but seems okay on open road, mechanic has had a look and no visible leaks and checked gaskets and all okay. What could it be?
The hoses may not be sealing properly. When you're in traffic, your car gets about 20% hotter than on the open road, so the extra pressure caused by trapped steam (or hot air at any rate) may be forcing the water out of a leak that is invisible at lower temperatures.
At least, that's what I found with my Astra.
How many times have you refilled the reservoir? It is possible that air was trapped in the system after the water pump replacement. After a couple of high speed runs the air may have worked its way out of the cooling system and into the reservoir. Some vehicles are more difficult to "burp then others. If you have filled it more than 2-3 times you have a leak. Ask your mechanic to test the radiator for the presence of combustion gasses which will indicate a bad head gasket. Did he do a pressure test of the cooling system or just look for drips? If doing a pressure test make sure to check the heater core as drip from it may stay in the heater box and not reach the ground.
The question mentions having 'belts' changed. If the problem was not there before the belts were changed, and the problem involves overheating, then it is very likely that one of the belts changed was the cam belt. If the engine was not timed properly with a new cambelt fitted this would cause your problem.
A few pointers:
1. Check your cooling fans, specifically: (a). Your cooling fan control module(s). You may be able to use VCDS to test for actuation (where applicable, not all models support this functionality). In this case, it's likely that your fan's not running at the correct temperature value triggered by the radiator cooling fan switch. (b). The wiring harness to the cooling fan's motors and control module. These tend to be fragile and can fray. Continuity testing can help identify broken wiring that can be then repaired and replaced as necessary. (c). The radiator cooling fan switch. This sensor can and will fail over time. Have it checked and replaced as necessary. These switches are not serviceable.
2. Faulty Coolant reservoir tank and/ or reservoir cap: I've encountered damaged coolant reservoir caps trigger the symptoms described above, where coolant loss and mild to severe overheating may be encountered. Pressure testing the cooling system is the only way to rule this out.
3. EGR cooler leaks: This is also a likely culprit, gauging by the age of your vehicle. A bit of disassembly is required to confirm this. Typical symptoms include subtle drops in coolant levels on trips with short drives.
4. Heater core(s) and heater core hoses: A very likely culprit, especially if coolant leaks are observed on the rear of the engine. This, like EGR cooler leaks, tends to happen in trickles and may not be obvious to spot. The sweet smell of coolant (G12+) will fill the cabin with A/C on, depending on the severity of the leak(s).