lp111 timer

Hello all, Im looking for some advice. I currently have a Drayton LP111 timer connected to my Vokera Linea compact boiler (no build in timer). I then also have a thermostat on the wall in another room. I am looking to replace both with the Nest heat link. Ive been digging around on google but...

My parents have just moved into a flat with a combo boiler and an lp111 timer. Heating not coming on reliably, although timer and thermostat correctly set up. Just went round, it was set to "timer" and was an hour into the third timer period. Temperature at roomstat was a couple of degrees...