Problem with fuel mileage plummeting on diesel Zafira.

My wife and I purchased our 2nd Vauxhall Zafira in March 2018. Our previous petrol one was getting old, but we liked the car and it suited our needs. As we're self employed we have to keep mileage records, and that car used to give is about 33mpg. The new one, when we got it, was on the same, 33.2mpg IIRC.

Within 2 months we'd got that to 42mpg, and it slowly kept rising. It topped out at 43.8mpg. We had absolutely no idea how someone was driving this car and only getting 33mpg out of it. It's a really nice drive. Needless to say we were happy as we were getting a lot more bang for our buck so to speak. At first I assumed the board computer was wrong, so I manually recorded the odometer and fuel and worked it out... It was accurate to about 2%. Close enough for me!

For the last few months the mpg has been dropping. When it dropped well below 40mpg I reset the 2nd board computer to monitor it. Although the first board computer is reading 38.7mpg on the last 123 gallons, the 2nd board computer is saying we've done 32.7mpg on the last 27gallons. And I've worked out the last 4 gallons myself and it's accurate.

So whatever was happening is happening again. Problem is we have no idea what it is. We've spoken to a couple of mechanics and the only advice we've been given is "use the premium deisel" and also "try Red-Ex" neither of which have made one jot of difference.

The car drives absolutely fine, but it does have one new oddity. When I'm sat at lights, car stationary but the engine is ticking over, the "instance consumption" usually says 0.1 gal/h. Maybe 0.2 gal/h. Occasionally this jumps up to anything like 0.7, 0.8, 0.9 gal/h. Yet I'm sat there, stationary, the engine sounds fine and the revs are sat nicely at 900 or so. When it does this, if so watch the fuel, I can be doing 25mph in 3rd gear, getting 15 miles per gallon. Only a restart of the engine will fix it.

The car was serviced and passed it's MoT last March, zero issues. Reg is DK63 AFK is anyone needs to look up specs (I wouldn't know what you'd need to know)

Does anyone have any ideas at to what's wrong? Cos I'm baffled.