Further update!
I'm wary this could turn into a blog-like-epic, but all the same, thought I'd post my latest findings - to see if anybody with a sharper brain can illuminate me.
So I'd replaced the IACV, with a pattern part, that didn't seem to cure the problem.
Further cleaning of the throttle body seemed to have a positive effect - in fact, I don't think I've had any instances when it's happened and blipping the throttle has pushed the idle any higher.
Before my code reader arrived, but after I'd had an abortive attempt at changing the CTS (speculatively bought a new one, as there's a Kia dealership literally 5 minutes from where I live) I'd done some more cleaning of the throttle body, as I'd had it all apart to try and get better access to change the CTS. I also adjusted / checked the throttle cable, and made sure the linkage and spring were all clean and moving freely. So rather than disconnect the battery (to reset the ECU - I wanted to have a clean-ish slate for when I code properly read the codes) - didn't particularly want the rigmarole of having to set the clock and the radio... - I noticed the fusebox has a dedicated fuse for the ECU, so I took the fuse out for a few hours.
After that, things seem to improve quite a bit, really, no real instances of it happening for a couple of days, and when it did occur, it wasn't that bad - about 1500rpms, blipping the throttle didn't push it any higher, turning on the air-con would make it drop right down, and in many instances, it just didn't do it.
At this point, my code reader arrived - worked perfectly, no codes stored, no reported issues displayed.
Any way, having that problem where you just can stop tinkering, realising I hadn't done as thorough job cleaning the throttle body as I'd liked - and after all the problem hadn't totally gone away - I thought I'd give it one more go, with an old toothbrush - working on the premise that things seemed to have improved since doing so in the previous instance.
That didn't seem to make any difference in the short term. However, yesterday, during my second 60-ish mile journey in it, the rev problem came back with a vengeance. When hot, and, say, leaving one motorway, stopping at lights / junctions, revs would stay at 1500rpms (minimum) and steadily climb - I think if I'd have left it to, probably to the rev limiter, because in a couple of instances it had got to 3k revs and was still going. Turning on the air-con - especially when just stopped / just as the revs started climbing could see it halt progress, and if timed well, drop down to 1k revs, but when the revs had already climbed to about 3k, it had limited effect. Also, when the revs were high, and I hadn't been that quick getting the air-con turned on, or it took a second or two for the ECU to turn on the compressor, selecting first, and gently taking up the bite to move the car (not leaving the clutch wearing away, though) was enough loading to drag the revs down.
When I got a chance to stop, I plugged in the code reader - but still nothing - no codes, and nothing else within the diagnostics on it, to point to anything.
When I'd finally stopped, I looked to see the throttle linkage and spring position, the throttle didn't look to be stuck open.
So now I'm puzzled, I guess the only thing that could have the revs climbing like that - assuming the throttle is closed, linkage and cable not sticking, is the IACV. I can't believe the IACV has gone bad, so can only think it's doing this under instruction from the ECU. The IACV doesn't seem to be sticking - because when stopped, revs can be around 1500, then start climbing. Plus I haven't seen the return of blipping the throttle pushing the revs higher, when it's happening. Although, truth be told, the revs are creeping higher all by itself.
Vacuum leak has been mentioned, but would it keep climbing like this (3k, 4k probably higher revs, if left to it's own devices) - but not comprehensively - if I was just parked up, turned on, let it warm slowly at idle, it would probably not do this. And wouldn't I likely see some error codes from such a significant induction leak?
Next plan of action is to await delivery of one of those USB ECU live data thingmies (ELM?) and run the software on a laptop and plug it in when it's behaving like this, and see what all the live data monitor-ables show - maybe that might give some clue as to what's getting the engine to behave like this.
Do the current symptoms make any sense to anybody - any other advice? And yes, I'm rapidly approaching the whole take it to a dealers and let them sort it out - but, dammit, for my own sanity I'd just like to understand, and even fix it myself.