1997 Omega 2.5 v6 fuel consumption

For the last couple of days the engine has been running really rough
when cold, it's done it a bit over the last couple of weeks but not for
long.

After startup from cold it's doing 8.5mpg at 30 and 23mpg on overrun.
As it warms up this rises and after a couple of miles it's doing low
30's and overun is 999mpg.

I'm assuming it's the ECU coolant sensor and since it's a pain to get
to I might as well just replace it but I have a couple of questions and
hope someone can provide some guidance.

I know I've got to take the top of the inlet manifold off but do I need
to take the lower section off as well? I can't tell from the pictures
in the Haynes manual. I need to know what gaskets to get so that I can
put it back together again.

I can also hear air hissing somewhere near the inlet manifold and if I
rest my hand on the metal next to the plastic strip with all the pipes
in the tone changes. Resting my hand of the plastic doesn't change the
sound. I can't see any split hoses and I can't feel any air if I move
my hand around amongst the pipes at the back of the engine.

I'm wondering if this is likely to be a gasket on the lower section of
the inlet manifold and whether I should replace these at the same time.
If it is an intake leak and it stops once the engine has warmed up it
could well be the cause of the problem but I might as well replace the
sensor while it's apart just in case.

Any thoughts?