So there's a few things a need to crack on with and a few questions!
Thinning the paints - is this to make the blending between shades better?
Yes, also if you use several layers of thinner paints rather than one layer of thick, the surface you end up with will be much smoother.
I live in a hard-water area so any recommendations for thinning?
You can buy distilled water very cheaply, or get some acrylic thinner from somewhere
Also shall redo the armour or just smooth out the highlights as they are?
The highlights seem to be in the right place, so you could just blend them in
Brighter highlights - on everything or just the armour?
Everything- contrast is important, the black could be highlighted up to space wolves/fenrisian grey (assuming games workshop) ,
the armour up to almost white at the very edges like on this ( http://www.coolminiornot.com/153114?browseid=2902327 ) or this ( http://www.coolminiornot.com/313167?browseid=2902327 ), the cloth up to almost white and the flesh up to a much paler colour, also for flesh mixing in a tiny bit of blue with the pink tones is good, makes it look more realistic
Should I do a wash on the armour too?
Doesn't look like the armour really needs a wash, you go down dark enough in the recesses as it is
Thanks!
No problem, most people on this forum will be happy to help