Any question answered...

I have an Alfine 8 hub and it seems a lot of the time when I ease off the pedals in certain gears, it re-engages about 2 gears lower and I start spinning/hit my knees etc as it takes me totally by surprise, it was adjusted perfectly (aligned the green dots), and it was doing it mainly in 4th gear, but I figured the dots may not be that accurate and have been constantly fiddling since, adjusting cable tension only seems to change the gear that this happens in, what is the problem? It must have only covered ~300 miles since new...

The various service instructions i can find mention red dots, yellow dots, and yellow setting lines, but not green dots. The dots are for attaching the 'cassette joint' to the hub. Cable tension is set by installing the clamp at a fixed distance from the end of the outer with the shifter set to 1, then fine tuning so the yellow setting lines match while in gear 4. Are they what you're referring to?

http://techdocs.shimano.com/media/techdocs/content/cycle/SI/Alfine/SL-S500/SI-6M20A-001-ENG_v1_m56577569830657998.pdf
http://techdocs.shimano.com/media/techdocs/content/cycle/SI/Alfine/SGS500/35Z0C-001_SG-S500-EN_v1_m56577569830613357.pdf

When you say adjusting the tension just changes the gear that slips, are you talking about large adjustments that change which shifter settings correspond to which gears at the hub, or small adjustments? If the latter, this could be a sign that the cable is not following the correct winding path at one end or the other so you're not getting quite the right cable pull ratio. e.g. the note on point 9 of the first service instruction, or maybe some build-up in the slot the cable's wound into.

How is gear 5? And 8? Could it be that gears 2-4 slip down into 1 (and 6-8 into 5)? That would be one of the roller clutches slipping, and re-lubrication might be the answer.

(My practical experience is limited to older Sturmey Archer hubs, but i've spent many fascinated hours on the web trying to understand modern hubs gears.)