[Pete L]

On every start bar the last two, the oil pressure indication has steadily reduced as the oil temperature indication increased.

On the last couple, there's been a little fluctuation towards the end of the process, and on the last flight the oil temperature indicated significantly higher than normal around 10 min.

The aircraft has an oil cooler bypass fitted.

My guess is it's the bypass valve - any other ideas?

[cockney steve]

Tried stopping the engine and having a quick feel of the cooler inlet, outlet, matrix and associated pipework?- but, yes, it does rather sound as though the cooler-bypass (thermostat?) is stopping the flow through it.,thus reducing effective volume and raising temps in the circulating part of the circuit.working cooler should have a detectable heat-gradient between the connections and the matrix also, as it dissipates the incoming surplus heat. the fluctuating would seem to back up your diagnosis, also.