If the overflow reservoir bubbles smell like exhaust fumes then the cylinder head is leaking. Hopefully it is a broken head gasket rather than a cracked cylinder head. A compression test of each cylinder will prove if its a cylinder head problem.

2003 Sirion SL 1300. And I love it.
Don't like the new shape much.

I hope Assasins suggestion works! because if it doesn't the next thing I fear will be the head gasket has blown (High Pressure in the system could point to this)
Or Cracked Cylinder Head.
Doing the Higher Miles and Overheating the engine could have caused this.

Fingers Crossed.

Bob F.

Sounds suspiciously like an air lock in the cooling system, if the fan switch is in air and not water it will not activate the electric fan.

Drain the cooling system, refill slowly with water, ensuring the heater control is in the hot position, and not cold; and run the engine until it is hot. Bleed the system thoroughly and top up, run the engine for a few minutes and then drain the water out.
Note the colour, if it is sludgy and brown the system is contaminated with corrosion, previous owners have not kept it filled with anti freeze.
remove as many pipes as possible at one end, push a hosepipe in and flush each part of the cooling system until it runs clean, do not forget the radiator.
Fill the system with water and cooling system cleaner, bleed the system and run the engine, follow the instructions with the cleaner.

Just one other thing to check! is the correct radiator fitted to the vehicle, i have seen radiators from lower powered models fitted to vehicles, these have less cooling capacity and lead to overheating.