There's only 2 components in them as you'll have seen. One is a small cartridge type bulb like in your dome light. Later versions may be a wire wound resistor. The other is a zener diode. Maplin sells these for about 45p and we know Maplin aren't that cheap these days, so they probably pay tuppence each for them.
The cheapo ones have a 27R 6W wirewound in, which is just about enough if the sender lead falls off, plus two zeners wired in parallel, which isn't particularly good practice . If you hunted around, you could get them for a couple of dollars from China: a lot less than £30. I've become a great fan of the steam-powered thermal strip type ...
The Heritage ones do have the VW part number stamped on them by a Chinese four-year-old. Surely that's worth another £25 ?
Apparently, according to "the whispers" on here he will deliver it to sunny Barnsley for food! I bet if you give him a go in the Barnsley red fluffy hoodie he'd fit the alternator too!!!
Joking apart the history could be interesting. I saw that the alternator was 021 903 021 12v 55 amp and wondered how it matched the engine. Noticing that the engine might have been ex-nato I was curious.
Fan housing hopefully if not there then it should be on the shank/shoulder by the oil breather filler... nice one thanks for having a look.