Well it shows low voltage connection on page 36 and normal voltage on page 37
you could use both but would advise the low voltage option,more protection for the boiler
you have following terminals
Main live and neutral this is the mains supply to operate unit and relay,this curcuit does not go into the boiler,just the unit
then you have
live in and live out
using the low voltage connection on boiler link cable from low voltage block on boiler to the two outlets on room stat controler (L in-L out ) it does not matter which way round really but you can use your tester to check if you wish to go by the book
if you want to use the mains supply connection, link live connection in control unit box, to live in connection then run cable from Live out to mains connection as detailed on page 37 (Terminal 50)
But as said above would advise using low voltage side and lable cable in box, low vlotage for future referance