A few hours of travelling through the V3M boards and still not sure but narrowing it down. Any thoughts or ideas really welcome and thanks in advance. Here's the facts...

1. Engine temp gauge sits here when it's heated up and stays there. It didn't do this for the first 6 months of ownership with the mileage 45-55k miles: 2.No evidence of head gasket sludge, just some light misting on the filler cap that's probably because it's spent most of the last 6 months standing: 3. No evidence of coolant leaks. However, it was low and has since been topped up. I used some Wynn's Radiator Stop Leak just in case but no change on the temp gauge. I did remove the radiator hose at the back of the engine (lower nearside hose) as it was bulging and may have cracked but not so (just covered in oil from an as yet undetermined leak above it). However, when the hose is fitted and I turn the engine off after heating up, there's a hiss somewhere in the engine bay that stops if I gently squeeze the hose - strange. 4. No rust in the expansion tank, radiator fins look in pretty good if not excellent condition and coolant looks a usual blue/green.

5. I think the thermostat is working as I can feel hot coolant going through the lower radiator hose on the nearside and coolant is flowing through the expansion tank.

6. Radiator fan comes on when the engine temp rests just above middle and stays on for about 45-60 seconds then cuts out.

7. Engine oil is low but just about acceptable.

So I figure this leaves the following options:
- Thermostat starting to fail and not fully opening
- Radiator fan not cooling correctly
- Engine oil not providing adequate cooling
- Slow coolant leak from a hose masked by an oil leak from another source or evaporating with engine heat
- Early onset head gasket failure
- This is just normal don't worry about it

Anything else that I've missed? Any other diagnostic things to try?

UPDATE: Added just under 1L of oil. The fan comes on just as the temp gauge tips past the middle and that brings it back down to pretty much dead on center. I'll do a full oil change at the weekend (Castrol Magnatec 10w40 A3/B4) as I reckon the oil inside is dead and useless. Thinking about it now it's probably not been changed for nearly 15k miles since the last service stamp 4 years ago and has been off the road for much of the 3 years before I got it.