Well it's safe to say changing the alternator belt and tensioner did not fix my intermittent stalling problem - it did seem like a long shot, but at least the engine is quieter! On a trip up to the coast the problem returned with a vengence. No fault codes still but a number of likely candidates.
The next thing to look at would be an air leak or a sticking/dirty throttle body. So I removed the throttle air intake pipe and found it just starting to crack in the creases, but it was not perforated just yet. That was a good find, so I decided to change that.
On removing the pipe I found a curious piece of plastic resting casually on the pipes / hoses below. And then the lightbulb moment occured - the last time I was under the bonnet I noticed it sticking out of the end of the idle stepper motor. I had pushed it back on at the time, but it was obviously very determined to escape from it's lodgings.
Bosch part number 0132008650
Replacement instructions here: https://www.alfaworkshop.co.uk/alfa_replace_idle_control_motor.shtml
This is referred to by various combinations of names. Idle Stepper Motor seems most appropriate, but I've also seen it referred to as a Throttle Control Blade so handle with care I guess!
Note: THIS PART ONLY APPEARS ON the GTVs/Spiders with CF2 TWINSPARK ENGINES, and at least one other car - the Fiat Bravo / Brava.
So there was nothing for it - I had to try fixing / replacing this part. With the cover (the bit with the curious star motif) coming loose, it was inevitably letting moisture and dirt in which probably wasn't helping.
Just one problem: this part is no longer available. But I luckily found one on eBay and made an offer for 10% of the average asking price of most other sellers with this item, and thankfully it was accepted.
Once the used part (from a Fiat no less) arrived I replaced this and the deteriorating air intake pipe.
I had to also reset the idle speed using diagnostic tools, but it can also be done a special key in ignition ceremony. I'm happy to say this has fixed my problem.
So before I jump into any more maintenance / improvement on this car I feel I ought to figure out what is going on here with availability of this thing and see if I can to get someone to manufacture more of them. I really don't want myself or anyone else to have to pay the extortionate amounts some people are charging for them. Especially as I have the old part from my car which could be used as a sample to make a new one.
I'll post an update if I have any success.