In reply to Removed User:
I have 8 radiators in my house, but you have 21 ??????? Surely you have a maintenance team to deal with these issues
In my lesser experience, the high kW is useful when having a shower, not necessarily for the rads. I'd size it on the DHW requirements, but then again... 21 rads????
Get an independent qualified heating engineer to give you a qualified opinion on your existing Boiler. If it is sludge a powerflush should shift most of it. BUT -- if you change the boiler and it was a sludge problem you'll just have sludge in a new boiler
Don't forget to add inhibitor before you fill it up again, you can squirt it in a rad by unscrewing a bleed valve.