Thanks for the info Lee, very helpful.
I've looked into the air-con high pressure switch in the past, I've found the part number and it's a fairly cheap part (for these cars!). It's located in front of the condenser so it requires the front bumper to be removed for replacement - not a huge problem as I need to remove the bumper at some point to replace a couple of front parking sensors anyway. I'm assuming this is the cause of the fans running constantly but I'm not sure this alone would cause the MPG to drop so much. Of course it would be useful to fix the fan problem regardless so I'll try to get that sorted soon.
I understand what you're saying about the secondary air injection. I was actually planning on fitting a bung to the SAI pump outlet hose to rule out any oxygen entering the exhaust from that area but after blowing through the pipe I assumed that wouldn't be the problem. If there's some sort of check valve as you say this could still be an option. I'll measure the pipe diameter tomorrow and find some form of bung and try again.
I had also considered exhaust manifold leaks but I thought I'd hear any leak if it was bad enough to be causing these problems so I didn't look into it any further. There's no obvious exhaust leak noises.
I'm thinking the problem is something along these lines as there's no obvious loss in performance, the lambda sensors just seem to be picking up excess air from somewhere and dumping in fuel to compensate.
I have already replaced the PCV valve as a precaution but there doesn't seem to be excessive vacuum in the crankcase, the oil filler cap feels normal while the engine is running.
Fuel pressure appears to be fine from testing with my homemade gauge, but it does have 2 fuel pumps so it's possible this be masking an issue.
Thanks
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