Boiler flame very yellow

[Gusty1]

My 12 year old Potterton 120L now burns pretty yellow and I can see small flecks glowing red hot on the bottom as I look in the inspection window. I am looking for advise re best course of action. Let it run as it is? Replace the heat exchanger itself? Get the heat exchanger (sealed unit) serviced? Should I look at a one off repair type contract as these tend to be a fixed cost and avoid any nasty surprise costs or would they not cover such an item as technically the boiler isn't (yet) broken? Thanks for advice in advance.

 

[Billy Bob Bob]

get a gas safe engineer in and stop using the boiler ASAP