Smudger – bold claim re the pellet boiler!
I’ve not tried theirs but they are not generally recognised as being one of the best. Okofen, HDG, Windhager, Froling would possibly de recognised as the big four.
Their oil boilers are fine, although not sure Riello are the best burners, expensive parts compared to others and you have to use a Riello spare. Saying that they are a good burner just haven’t found them to be significantly better than other makes.
But as I said if you are just looking for hot water then an unvented cylinder will be cheapest and easiest. My local City Plumbing have got a couple of direct units that they are selling on a make them an offer so could be quite cheap.