1997 Ford KA car revs when clutch is down

My KA has this problem where once my car is (roughly) warmed up, about 2-3 minutes into any journey, every time I press the clutch down the car starts off with a very low revving sound, and then it just keeps rising and rising, as though someone is pressing the accelerator whilst the clutch is down. If I'm accelerating (say, from 1st to 2nd gear, 2nd to 3rd gear, 3rd to 4th, 4th to 5th), it will rev itself indefinitely (until i release the clutch)..however, if I'm decelerating (like for a junction), I take my foot off the accelerator, wait a second, put my foot down on the clutch and brake...and it will rev but only until I've braked down to a speed of about 10 mph, when it will die down and be normal. At first I thought it might be the throttle sticking, so I used some WD40 on the throttle linkage (the bit with the springs around it) and that wasn't the problem. I have checked that the clutch cable and the throttle cable aren't tangled in the driving seat footwell. That wasn't the problem either. I've seen numerous suggestions about the Idle Speed Control Valve..and the symptoms commonly associated with problems with the ISCV are shockingly similar to my own...So I bought a new one and fitted it..to no avail. I then saw a post (on another forum) about the throttle position sensor voltage being too high, but neither myself or my dad (who used to be a gun-mechanic in the army) knows how to test this properly. If possible I would like suggestions as it's rather embarassing and every time I put the clutch down to change gear it just sounds like I can't drive for toffee. I would appreciate any advice/help/tips you could give me on this, it's just awkward in every way.