As hopey said but I'd fill up the joists i.e. if you have 100mm joists, get 100mm insulation, if you have 150mm joists, get 150mm insulation etc. and then the 170mm laid at right angles to the insulation between the joists, over the top.
What you do need to allow for though is if you have walkway boarding down the middle: You will need to allow for additional battens on top of the joists to accomodate the extra thickness of insulation.
I was under the impression that you could only get a grant if there was no insulation present.
And +1 for Rockwool roll