Well.. I managed it in the end...
and in hindsight it wasn't so tricky.
The biggest problems were the never-been-undone nuts and bolts, and working out the right order to do everything.
The alternator will only come out between the engine and the bulkhead, via where the offside/right driveshaft lives.
To remove the driveshaft you first have to remove the support bracket. It's held in place by some nuts and an engine mount, so the first job was to loosen the mount and jiggle it out of the way. It's an 18mm from memory and very dry/rusted on my old beast so I was afraid I would damage the threads... but half a can of Plusgas and another of WD40... it survived.
Next job was to remove the nuts/bolts. The nuts are 17mm nylock and no bother, although on mine one of the studs came out of the block, instead of the nut coming off the stud.
The bolt is a 15mm hex and is very inaccessible. The support is in the way from below... so the only way to get a spanner on it is from above, but then there's very little room to turn it because Fiat put the engine and the bulkhead just where the spanner/ratchet wants to go.
I'd rounded the nut off a bit (unable to get a straight angle of attack on it) so I was preparing to introduce it to Her Majesty (Grinder, queen of the workshop) but just for this job I bought a 6-sided Hex socket which gripped the bolt head better and that broke the stiction.
Driveshaft off needs the gear oil drained. Luckily my old mum had just that day bought a brand new 5ltr plastic bucket, so I used that to catch my oil. It's only 1000 miles old, since I changed the nearside driveshaft so it would've been a pity (£25) to bin it.
Driveshaft just came out when the support bracket was removed.
The alternator was similiarly truculent. There's a 15mm pass-through bolt above and a 17mm pass-through bolt below. The 17mm bolt came undone okay (you can get a spanner onto both the nut and the bolt head) but the top one was way more fiddly. Again, space to get a spanner onto the bolt/nut heads and then turn it is the biggest problem.
The spacer fell out when I removed the bolt. If I hadn't read Davren's post above I might have panicked.. but I still didn't see where it fell out from. I worked out that it fits into the void behind the outer face of the alternator bracket, on the chassis rail side of the assembly.
The alternator is held in contact by 12mm and 8mm bolts (positive and earth) which were easy to undo.. but disconnect the battery first as the connections are "hot" and the beast sparks a bit if you accidentally earth yourself while undoing the nuts.
The alternator I used was a Denso DAN501 ... 105A @ 14V and just £105 for my old beast's standard Fiat (but also made by Denso) part 46782213. I noticed that the standard one used a Magneti Marelli regulator/rectifier whereas the Denso was slightly different and unbranded (probably Hitachi?). Marelli had a cover over the electical connectors whereas the Denso/Denso didn't. I doubt it's too serious.
Putting everything back.. I ran out of energy trying to faff about with putting back the alternator top-bolt/spacer/washers/nut with one hand up the back of the engine and the other steadying the alternator... so I removed the flywheel pulley and the aux-belt tensioner to open up a bit of space through the wheel arch. It takes about 5 minutes longer... but it saves the time immediately and no arseing aboot.
Refitting everything else was the opposite of dismantly... just remembering what I'd removed in what order, and tightening all the bolts that were just loosened, including the gear oil drain plug and the bottom engine mount... then re-filling the transmission oil (degreasing mum's bucket so that she couldn't tell the difference) and topping that up to the correct level. Aux Belt is 6PK1270 for me (air-con model).
So far, the beast is running smoooooth. I didn't realise quite what a racket the alternator had been making... so worth all the aggro. I think..