A small amount of mayo on filler cap is normal. It's a cold point on the engine, the reason you did not see is this in August is due to temperature... Short trips make this worse.
As for the high temp could be:
- faulty sensor (although that is usually rather complete failure or it "jumps" all over), can be tested with ohm meter (10£ multimeter from DIY shop will do, see attached photo for target values, sensor is in slightly different place on 1.4 and 1.6 blocks)
- thermostat, quite likely tbh, feel if both hoses are hot or bottom hose is at least very warm when it shows over 90 degrees. in this weather you'll have to drive 20 miles to properly get engine warm unless something is faulty.
- blocked radiator, same for diagnosis as thermostat (or takes hose off and see if water comes out the bottom if you pour it in the top)
- impeller has separated from shaft in the water pump, if it's this means you need a new one (plus cam belt, tensioner, etc)
and well, could be head gasket.... but would first go through others unless it's spitting with white smoke and goes through coolant as fast as petrol....
