Thanks for all the replies.
@JohnMo
Well "pretty well insulated" is a big statement. I still have (insulated) shutters which can let in cold air as they're still shutters after all. The walls have 25mm insulation (plus drywall and a gap) and ceiling is 100mm. Floor is only a 6mm sheet plus rubber tiles. What I want to say is that it's "insulated" to a point where it's comfortable to work with the heating on but when it's cold outside my heat pump works at minimum all the time. I have installed some thick curtains over the shutters to keep the heat in but as I said I'd like some fresh air coming in. Installing extractors means even more cold air would be drawn in which is going to translate to more energy required to heat the space. I am a DIY'er, I have no problems installing the MVHR by myself. It doesn't have to run all the time, mostly when I am using the space.
Thanks for recommending BPCVentilation, I'll take a look!
@Adsibob
I have space above the ceiling, I can run all the ducts I want there
Soldering fumes is not a lot, but over many hours it can build up. This is nothing like welding fumes or cigarette smoke etc. I was thinking of just a single inlet and a single outlet to be honest. Ceiling height is 240cm.
I did stumble into the Vent-Axia - it's still £380 which is £130 more expensive than the one from Ebay. With pipes and fittings (I'll do the install) it's way over £400. Obviously the old say "you get what you pay for" is very well in front of me
Thank you!