Hi Bettyk,
I would guess that you have identified the most likely cause of the issue, which is the holes caused by the downlighters. You appear to have approached it logically, Identifying the problem (cold/draughts), identifying the cause, after ruling out other possible causes (lack of insulation). So I'd have every faith in you identifying the correct solution. As you probably know, LEDs don't generate anything like the heat of a halogen, and if you can fit them including the airtight covers, you're on a winner.
I had a similar issue, but was able to access the existing downlighters from the attic. So I just fitted fireproof hoods. As you don't have this option, the one you referenced seems to be the best (or better) alternative.
I'd guess this is the product
[broken link removed]
And at €15 it's a pretty inexpensive solution (or €40 for 3).
I presume you will be able to use the existing downlighters, and buy new LED bulbs. Be careful if you have a dimmer switch, as not all LEDs play nice with dimmers, and dimmers have to be compatible with LED bulbs.