My Rover 45 lost power..OK on flat, useless on hills or starting off?

I told my mechanic the car had lost power and requested a service, new filters etc. I also asked him to replace the accelerator cable as I thought there was too much play in it. Got the car back, no new cable.."Not needed" bill for £90.
A month later, I took it for a diagnostic and was told it needed an air flow meter and a new belt at the fuel pump. It was due a timimg belt so I had al the belts done at the one time..£190. The mechanic said it still needed an air flow meter so I took it along and got one fitted...£96. Still no improvement!!! I went back to the diagnostic and explained that there was still no power. The chap lifted the bonnet and said he couldn't think of anything else. I mentioned the accelerator cable...!!!*****
When he tried the pedal he said we had found the problem. New cable fitted..£15!!! Car goes like a bomb.

How is it that an electronic diagnosis machine cannot pick that up???