Hi,
It would be really hard to tell without seeing the boiler & the installation parameters.
Potterton's are not a very common oil boiler in Ireland but they are a very good & efficient boiler as far as standard efficiency boilers go. They are usually in the mid to high 80's% for combustion efficiency. The baffles in them are unusual but effective & I would ensure that these are correct & in good condition. If replaced in the past with "something else", this could be an issue.
The boiler should be sized for the installation. A proper heat loss calculation can easily be carried out for each room & the boiler output can therefore be matched. This must be done accurately.
The system pump should be sized accordingly. Often under-sized.
The radiators should be checked for circulation issues, such as balancing & sludge content. I use a thermal image camera for this.
Are all the radiators heating up correctly & do not just feel them at the top? Check the temps at the top & the bottom of the rads. Check the flow & return temps on each rad pipework. Check the output of each rad in relation to the calculated heat loss of the each room.
Once you have all this information, the answer you are looking for will surface.