Problem began with struggling ignition a few times, then failing with a single "click/clunk" noise. Battery was ~7 years old so assumed that was the cause, but a new battery is doing the same thing.

Wiped down terminals/connectors, tested engine/chassis ground, checked 7.5 PCM fuse, checked for error codes from ODB (none), no dash errors, then changed the battery in the key fob to tick that box and it worked straight away!

Problem solved I thought, but after four 15-30 minutes journeys of the car working and noticeably better for the new battery, it went straight back to ignition failing with the single "clunk/click". After trying for maybe a dozen times total, the noise has now been replaced with a distinctive faint whirring... lasts about a second then goes to a slightly higher pitch for another second.

Is there anything left for me to try before cleaning the solenoid connections and/if replacing it? I have charged both car batteries multiple times along the way, no difference.

- Proper new Duracell batteries in the key fob as I was an replacing old cheap used battery with old cheap new battery anyway, or was the battery change success likely a coincidence?

A couple of things I found while Googling:

- On/Offing a C90 fusebox connector seemed to work for a few people, but I could find no explanation of where/what that is.

- Found something about two relays being identical and swapping them, only the R11 Fuel Pump seemed to fit the bill so tried anyway, didn't work.

Thanks.

It would seem that the most probable cause is a faulty starter solenoid or the control wire connecting to it. On your model the starter motor and starter solenoid are a combined unit and are replaced as a complete assembly. 

I’m a bit out touch with starter motors on modern cars, my trouble shooting skills are based on 70s/80s cars.  How many wires go from loom to starter motor?  Is there just one thick one and one thin one? If so, thick one is permanently live from positive battery terminal.  The other is live only when trying to operate starter motor.  So if you take that thin one off the starter motor , using a voltmeter , is it live when trying to start the car?,  when I say starter motor I mean the combined starter  motor / solenoid unit.  The wires I am talking about connect to the solenoid part of the combined unit. 

More… in the old days if the starter motor was faulty it could be repaired by replacing the carbon brushes &/or recutting the grooves in the commutator , depending what the exact problem was.  I did a few where it just needed the grooves recutting , easy job .  Perhaps they are all sealed up to stop anyone trying to fix their own starter motor these days.   I did have one car where the solenoid stopped working as damp got in it and jammed the sliding part with corrosion.   I had another where the link cable between the solenoid and starter motor (about three inches long) corroded through

But, if it was that short link cable, or the carbon brushes or commutator grooves you should still get a nice click from the solenoid. 

3 hours ago, isetta said:

But, if it was that short link cable, or the carbon brushes or commutator grooves you should still get a nice click from the solenoid. 

At first I did get the click. Now I do not.

I'm confused about why it went straight from dead to perfect to dead again, it seems usually you'd expect erratic struggling.

58 minutes ago, EndorLyle said:

it seems usually you'd expect erratic struggling.

Why?

If you have a broken wire to the starter solenoid or the actual solenoid coil has failed.

Agree with all above, and just wanted to mention that the 'key fob' is a red herring. As long as the new battery is pushing out 3.3v (no matter what brand it is), it won't cause any 'starting' issues. (Even if the battery was dead, it would still start the car as there is a built in chip in the fob that will deactivate the immobiliser without any power as it is proximity based)