Excellent advice agile. To many times a heat only boiler is ridiculously oversized. Our 150m2 4 bed house with bathroom and shower room has a 10.5kw heat loss and a 15kw system boiler paired with a 250 litre cylinder. We've never had any problems with it.
OP may be worth looking at a Worcester...
.There is no biofuel in UK kerosene. The problems that Danfoss are experiencing are a result of low sulphur in the oil and do not affect Riello pumps. Danfoss have now reengineered their pumps to solve the problem.
Do not add any additives they may do more harm than good.
Your pump problem...
If you're looking for an oil fired combi it has to be a Worcester Heatslave. The latest model is streets ahead of the rest and there is a 5 year guarantee available.
The reason the Intergas is so simple is that it is a backwards design. All other manufacturers have move to a seperate plate heat exchanger as it provides better hot water performance.
I'd look at another manufacturer rather than Clearview. Their stoves are overpriced compared to equally good products from the likes of Charnwood and Stovax and the efficiency of the stoves are low.
Sounds to me like the boiler is operating normally for the type of boiler it is. In winter the heat exchanger will heat up to the central heating flow temperature. When you open the tap the water will be very hot at first from the heating temperature and then drop down to lukewarm.
This is...
Why is it the boiler maker's problem? OP buys boiler intended to be used in UK and takes it out of the country and runs it on a fuel that it was not intended to be used with.
Now if this was a modern boiler design with a plate heat exchanger it would be less likely to scale and it would be a quick and simple job to swap the plate.