According to a post here:
//www.diynot.com/forums/viewtopic.php?t=152468 , at your boiler, 3,4 and 5 are Live, switched live, and Neutral.
At the nest, 2 is Common, 3 is NO (call for heat) and obviously L and N are Live and neutral.
So, connect boiler 3 (live) to Heatlink L and 2.
connect boiler 4 to Heatlink 3.
connect boiler 5(neutral) to Heatlink N
I managed to get two cores in the live terminal of our heatlink to link across to 2, but I was using the .75mm (I think) cable which went to my old stat.
You have probably worked this out, but the actual nest controller just uses the supplied mains adapter for power, and talks wireless to the heatlink, but if you already have a cable from where you are putting the nest to the heatlink you can use the heatlink T1&T2 terminals to supply the nest controller instead of using the mains adapter.