I would replace oil separator and breather but feel I may be wasting my time and effort. I was thinking of buying a low mileage fabia estate diesel similar year as these seem to have a far better engine noone sings praises of the VW htp 1.2 cam chain engine it appears fragile.
On our Fabia later we had apparently correct temperature readings consistent with engine warm up etc. but (often a few minutes after start) the warning light for engine temperature came on and stayed on until next engine stop. Coolant level was correct. It turned out to be the coolant level sensor in the coolant canister.
Fabia Mk2 > Skoda Workshop Service and Repair Manuals > Drive unit | 1.2/47 kW; 1.2/51 kW Engine | Enginecrankshaft group, pistons | Disassembling and assembling engine | Valve control drive - summary of componentsNMFzIEs.