What is the the unit below the vacuum pump (dismantled in picture) ?

It has a three pinned connection on the side, each with one push on caps, similar as spark plug caps.

I think I may have mixed up the positions of these caps while dismantling the vacuum pump. Are their positions interchangable, or do they have fixed positions?

Car is 2002 Skoda Octavia 1.9 tdi 66kw

Given your location, I would say that it's a coolant heater, the plugs will be what heats the coolant.

if there are 2 then I guess it doesn't matter,  I don't know if it fires 1 then no2 or both together,  even 3 wouldn't make a lot of difference,  but three may be on 2 relays  because of the load

^ Bang on, I think, but all three glowplugs will be the same, I'd think, so which one/two are being energised by each relay won't make a jot of difference.

Many thanks all