I would be looking at the sensors first and pcb last. Most sensors cost £10 or so. If you can get the data from BaxiPotterton you can prove them. They are thermisters which vary in resistance depending on heat. Heat yours in a pan with water and a thermometer and prove it follows the chart. I have the resistance data for just a few but not your Solo. A lynx for instance ranges from 0 deg c 33.6kohm through 50 deg 3k5 to 99 deg c 716 ohm. Make your own graph. The Myson Midas otoh ranges 64k at 10 deg through 10k at 50 deg to 3k at 80 deg. The Puma ranges from 7k (max temp) to 15k (minimum temp). So you see thermioster values are variable across boilers.

You could empirically test a boiler with a variable resistor and thermometer, see at which temperatures which resistance alters the modureg. Then test your Thermister from that data. With a combi most manufacturers use the same thermister for heating and hot water, so you just switch leads to prove that fault (don't forget to switch back or you'll have hot water and heating that doesn't modulate). A friend of mine did leave one in a house with the leads switched for a year until he next serviced it when he found his mistake. Householder hadn't noticed anything wrong.

Always prove all the little things are working right before you start to blame the pcb.