In the last 2 months I've noticed a marked drop off in power (top end
power). Its not as noticable just driving the car alone, but with my big
trailer, I used to be able to pull it happily at 60mph but now struggle
above 40. On my last 1000mile trip at the outset all seemed basically ok
though a little sluggish but over the outward 500miles the performance
dropped off remarkably.  I removed the air filter to allow me to get back
and which improved performance, but it wasn't dirty/blocked etc- and its
still not right. I usually get 25mpg when towing but have been getting 15mpg
recently.
The engine had a basic service in July  (oil+filters) and after that was
doing 39mpg on a long gentle run (w/out trailer) (the best I've ever got out
of it) and even now the engine idles and runs very smooth and although near
winter still 28-30mpg. Car has done 50000 miles.  I have run a tank with
cinjector cleaner recently too.

When you push the revs above say 3000, 1) you need to give lots of right
foot  and 2) its much more lethargic than it was.

So   what may cause this?   my thoughts are:

plug gaps widened?   they are due for a change, but  no misfiring though-
always starts fine
exhaust problem?    no leaks and sounds just the same but wondered if some
internal baffle may have failed
Sensor gone off?    perhaps a sensor is giving a strange reading making the
mixture far too rich?

I'm taking it to the dealers to have a diagnostics check done, but I don't
want the dealer changing and tinkering with allsorts unnecessarily.

Cliff