wrote:
I recently decided to re-insulate my loft, and and I found that there
was already approx 100mm width of existing insulation between the
ceiling joists. The existing insulation looked quite old, and the
joists were completely boarded over.

As the loft space isn't paticularly useable for storage (it is quite
pokey and the access hatch is tiny), and partly due to laziness, I
decided to lay new insulation (170mm width) over the top of the boards.

My question is:
Should I add another layer of insulation (100mm width) over top of the
170mm layer to bring it up to the recommended width of 270mm, or is
this unecessary due to the presence of the old insulation beneath the
boards, i.e. do layers of insulation need to laid directly on top of
each other for them to be effective?

Cheers,
Rick

I assume you mean should I add another layer of insulation (170mm thickness)
over the top of the 100mm layer to bring it up to the recommended thickness
of 270mm. I would say yes, and leave the boards where they are.