Thank you all very much for your replies. They have given me a lot to think about. I have done a fair amount of DIY over the years (plumbing, wiring, tiling etc) but have never undertaken what I would consider to be a building project ... hence probably a number of stupid questions.
To answer...
Thanks, smaller joists at right angles would definitely be a solution.
If there is any possibility of laying boarding directly (presumably cheaper and also less effort) that would be my preference.
I live in an older house (1841) where the upstairs level currently has no floors. The ceilings in the lower habitable level rest on top of the support beams/joists.
My ultimate goal is to convert the upstairs level so that it is habitable, but realistically this will be a multi-year project...