Update (with huge thanks to the ever-brilliant Ray at Northway Porsche who helped over the telephone): fan was not engaging at all, whether at high or low speed. The mechanic jumped the circuits and confirmed that the high speed only was was engaging, meaning that the ballast resistor needs doing (relatively common, I gather?)
The fan is not coming on at all though, meaning that the fan switch next to the oil radiator also needs doing (I gather :dont know: )
Anyway, parts ordered and should hopefully be back to normal service very shortly. Thank you all for your comments and advice!