It would appear that the Air Filter is not changed at every service on the Octy tdi , would i gain any better running if i did this myself evry 10/15k miles , the old one has been in for over 25k now .

Almost certainly yes. I vacuum out my cotton air filter every month or so and scrub it clean about every 8000 miles.

for the cost of a std fillter its well worth changing at 10k after 25k it must be nearly blocked !!!!!

Skoda used to recommend every 20,000 miles, but the air filters must have suddenly got twice as good overnight without changing part number, as when they included 3 years 45,000 miles free servicing on the cars the change interval went up to 40,000 miles.

Some OE paper filters have a layer of foam on the surface of the dirty side to prolong the filter life, but the one in the Fabia's not got it so I wouldn't have thought that was the case with the Octavia either.