Still for me its a thermostat issue. Designed to fail open they often get sluggish and tired in function. I think it tries to open but this happens only partially so the first time the engine reaches full temp the stat opens as far as it can. Water flows into the rad and cools to exit at the bottom hose, engine cools and stat closes. Now as the engine re-heats, the next time the stat opens,but not as far so you get water flow into the rad but slowly. this cycle carries on until the stat opening function weakens further and the radiator through put becomes slower and slower. The engine now holds onto its heat for longer but does eventually cool enough to allow the stat to close but soon you build retained heat in the engine which doesn't dissipate enough and quickly builds again. The stat by this time is at its weakest opening function. You might not even be getting enough heat through the rad to even trigger the fan heat sensor! A properly functioning system when the stat opens and reaches the bottom hose... well you ain't going to be able to hold it! To maintain an effective heat drop from water leaving the block to that returning is in the region of 10'c this enables the engine to maintain the optimum operating temperature band. This is why fitting too large a rad is not a good idea if it can cool the returning coolant at too low a temperature.
Get a new stat and gasket, then remove the old stat. Try not to damage the gasket/seal and refit the housing without the thermostat. Now run the car and see what happens. You should fail to reach operating temperature. check the flow across the radiator header tank, you should clearly see movement, if not the water pump needs looking at.
If all seems well I would now take the opportunity to flush the system. Get a flush product, Halfords sell Prestone and it's pretty good, in fact having tried many over the years I think its the best and its also suitable for alloy rads. After emptying the flush solution repeat a couple of times more with plain water to fully clean the system out.
When this is finished refit the new thermostat and gasket. Check the engine spec for the correct temp stat and believe this more than the indicated opening temp stamped on the old one. At this time go around all the hoses and squeeze them looking for weak areas/failing hoses, It would not be the first hose I have seen which was suffering from an internal delamination of the internal lining and this can restrict flow, in your instance pay particular attention to the top hose. Refill the system and see how it performs. If when half way through filling you can't get anymore coolant in and you haven't got the full required amount in just start the engine and the pump will draw it in, then continue filling until full. make sure your cockpit heater control is set to max heat and fan on first setting. As engine warms up you should feel warm air entering the car confirming the heater matrix is full. When system is full to the header tank cold or lower level mark put the cap on and run for a check.
