Thanks for all the replies so far.
EcoWool:
Eco-Wool Loft Insulation
I really recomend it as its easy to lay and itch free. Has a U value of 0.12 which when compared to the others on the market was good.
The 10cm variation is a bit odd TBH because I have tried to make it as consistant as possible. It may be that it needed settling so I will measure it this weekend.
The boards are standard loft boards and cover a part of the loft around the hatch (which I have changed for an insulated one). I went over joist level and then compressed it a little when I fitted the boards (which are tongue and groove for a tight fit) Around the edges I have overlapped approx 200mm of ecowool. I think when I have some money I will lift the boards and put down insulation board on top of the ecowool layer and then put the boards over that.
I think you are right about diminishing returns though.
Its frustrating that in a house only 10 years old it so woefully inneficient