What’s best here guys? Nikwax? Grangers?

Neverwet if it ever is available in the uk?

I yet to see any review that actually suggested one was better than the other – as in there both highly recommended.
I use Nikwax as it tends to have better availability & be slightly cheaper, I’m happy it does the job.

I just use the Grangers spray on stuff (and do it whilst it’s still slightly damp coming out of the wash). Seems to work pretty well but you really need to do it after every wash, fortunately I get 3 or 4 rides out of a jacket between washes else would be too much hassle/expense.

Nikwax doesn’t use fluoro-chemicals while Grangers does, if that bothers you. My experience is that they both work well. I’ve never seen a really objective review of one versus the other though.

You shouldn’t have to re-apply after every wash btw. You should be using pure soap or a soap-based technical cleaner rather than normal detergent.

And quite often heat treating with a tumble drier or ironing – follow the care label – will revive the water repellent treatment on the face of the jacket

Can you use that sort of stuff on ‘special’ fabric like Goretex / Hy-Vent stuff?

Been idly thinking of getting my North Face jacket done, but sending it off to someone to do it professionally; waste of money?

Yes, this stuff is specially made for the likes of Goretex & eVent (not familar with Hy-vent), and I throw my other unbranded wet weather gear in to & it ain’t hurt it yet. Obvious refer to the manufacturers website for specific info on what they recommend, but most will suggest one of these two normally. They would suggest using the a ‘tech wash’ first before reproofing too.
I’m unaware of professional services that offer cleaning/reproofing, though I can understand why you’d think seriously about using one for an expensive bit of wet weather kit.
But for the like of ‘most’ our kit get so dirty regularly, being able to do it at home is major benefit.