Over the holidays I insulated my loft with roll. I wanted to start with a clean base so basically used a dustpan and brush and a few bin bags to get rid of any loose items which were between the joists.
I'm not sure how much your Dyson would appretiate sucking up loose fill. If you can stand in there okay and can make up some boards to stand on then a shovel would be ideal. I couldn't, hence the dustpan and brush. Luckily, for some reason, there are 5 doors in my loft, which i used to sit on whilst doing it.
25mm is not that much though, if it is even, so perhaps you could just lay over the top. How high are the joists? Mine were only 80mm so I used 100mm thick roll between them.
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