Yes, eventually I insulated the lot with rockwool and as HERTS P&D said, I don't regret it.
The noise has been completely cancelled, unless someone stumps their feet upstairs, and the floors can be heated at different temperatures in minutes.
I've noticed a sharp reduction in gas usage, mainly because of the now very short length of CH pipework, but also, the heat remains for hours after I turn it off.
Example: come back from work and I have 16⁰ on the ground floor.
Heating on and within an hour is at 18⁰+.
Boiler stops running and it takes a good hour before the temperature drops by half degree.
That's in freezing conditions last winter.
Just now, we go to bed with 19⁰, boiler off and we wake up still on 18⁰. (But it's not that cold yet).
So, yes, I reccomend insulation.