Webby mate, we can't really answer, we've got no info.
What's the fuel economy figures?
What's been done so far?
What does tech 2 say?
First shout on fuel economy IMO would be multi rams, cam belt kit set up, blocked exhaust, old fuel filter.
Then I'd paper clip it for codes.
Then I'd get it on tech2 with one of the guys here for possible maf, air leak, or O2 senor problems depending on the readings.
Only then would I look at changing sensors if tech 2 showed evidence of a problem.
If I needed to swap them out, I'd get genuine Bosch from a factors. £51 each last I looked, but probably gone up since, for a face lift one.
But then... How much, more or less, of that has been done already...? 
sorry chris, i keep assuming people have read my many stoopid posts
this was my original techII read out...
O2 BLMs:
Integrators, around 100 (low)
Idle (B1 - 190, B2 - 160) (high)
Partial Load, around 130 (good)
Air/fuel ratio (Rich/Lean) seemed sluggish at idle, flipped at about 1Hz at 2000rpm, lazy lamdas, both banks.
Reset BLMs, took for drive, all around the 130 mark, except at idle, when integrators crept to 150ish, with Air/Fuel showing lean until then, then flipped at 0.33 - 1Hz
Seemed slightly down on power to me, although Webby said otherwise. MID readings for instantaneous consumption seemed quite low - I'd expect mid to high 30s average when cruising 50-60mph, reality was mid/high 20s. MID has correct 035 config, and readings seem to match Webby's experience of it using more fuel recently. This leads me to suspect that the engine is really using that amount of fuel, rather than leaks (which I couldn't detect, or smell).
After test run, brakes still cold. EGR can get to about 55% before the thing really splutters. Injector cut-off exactly as expected.
MAF, at idle, with A/C off, 9kg/hr.so far ...
replaced fuel filter
i've vacuum leak tested (no vac leaks i can find)
voltage leak tested (visual inspection and with bit of water... no sparking/arcing that i can see)
checked for exhaust leaks by me and by garage. no leaks. emissions test also came back perfect.
replaced MAF sensor with one cleaned with lots of electric contact cleaner...seems at first glance to have improved the MID readings but i'm no expert and will be doing brim to brim test as soon as i can afford it

also seems to be more responsive.
purchased spare fuel rail/inlets/injectors and will be taking to get them tested and putting that in after to rule out injector leaks
on markDTM's advice i have a good known disspac (thanks mark) that ill be swapping out this weekend
IF after all the above i can only think of checking timing, CTS, stat and o2 sensors.
sorry again for the ''constantness'' but i really want to get the best fuel economy as possible
