The ceiling boxes can get a little bit expensive when you have so many lights to cover. You can make your own but I imagine you can't be bothered when it comes to making 36 of the things! I made my own and they do the job absolutely fine.
The only thing they really do is give the lamp unit an air gap from the insulation. They don't have any fire rating unless you get the Fire-Rated intumescent hoods or have Fire-Rated lamps which if you have low voltage I imagine you don't.
I insulated my loft very cheap with 250mm Insulation which was actually deeper once it finished expanding after I rolled it out! I got four huge rolls for about £40 from Wickes. The insulation is subsidised so you get it cheaper - you have to say that you are using it for a domestic property which isn't a new build and I think the max number of rolls you can have is 10. I can't remember how much is in a roll but it is about 6-8m2.
Maybe now is the time to change out all your low voltage lamps for LED? About the same cost as a Fire Hood

That way you will get your savings in Electric and Heating!
The repayment time should be pretty quick for you... especially because you have oil heating and not Gas. With the price of Oil fluctuation as it does and depending on how many rolls you have to buy - I don't think it would take long to recoup £100.
Turks.