Thank you EVERYONE for your time, it really is appreciated.
Agile, Im over the moon, you have solved my problem and I wish I could buy you a drink (if that's your thing).
I had brand new batteries in the timer and they are Alkaline. This removed the low battery symbol so I thought all was well. However, these batteries were bought in a discount shop and are not a well known brand so on your advice I have just changed them for a couple of Energizer MAX batteries that I had at home and boom, boiler is reacting to the timer to turn on and off when required (tested 3 times).
Thank you so much.