Would it also be worth trying a second hand Bosch MAF or is it likely to be the lambda (or both) that causes the issue?
Not really as you have no idea if the 2nd hand MAF is any good. Two things I found to affect fuel consumption the most are lambda(s) and thermostat/temperature sender. (that's not ruling out a lead right boot!). Putting a non-standard lambda in can be a pain .. use Bosch, avoid NTK.
That is, if the air filter is clean and tyre pressures good. Can't imagine a problem with the air filter as the car has regularly had its service items replaced and it's only done 48,000 miles. How easy is the air filter accessibility to check it?
Ah. You'd be amazed how many invoices say "new filter" when it's obvious the cover hasn't been removed .. TS filters are a PITA to work on as the bolts tend to snap off .. and once the top cover fails to seal properly, that can cause issues.
As I say, the car idles beautifully and drives perfectly. Perhaps I'm creating problems in my mind but the mpg readout on the trip computer is definitely at 24mpg :-(
I don't want to rock the boat if it risks ruining how it feels!
Don't try comparing a TS petrol with a diesel .. diesel will always win!!
My father-in-law has a Snap-on scanner (very expensive piece of gear) for diagnostics etc but the 156 doesn't appear to have the same plug-in socket as the 159. Guess it's because it is older.
Should do .. it's behind the fuse box cover on the right side of the dash! Its a standard EOBD connector on that 156 and the 159. Although not all cars are as "standard" as they should be when it comes to diagnostics .. MultiEcuScan (MES) is still the preferred diagnostic on 156s.
To reset the ECU properly you need MES. You can leave the battery disconnected but it doesn't clear all the "learned parameters" ..
And like the temperature gauges in the 156, the MPG computer can be misleading .. I do the brim-it, drive it, brim-it calculations .. my 2.0 has been up to the high 30s before now. I regularly get low 30s even with "spirited" driving ... how else do you in an Alfa?!?