The C36 is absolutely NOT related to the C25 Celsius! It just happens to be made by the same company!
The electronics, burner / heat exchanger design and internal layout are COMPLETELY different (and better, IMHO). And it's a combi, while the C25 is a system boiler.
I've installed several and have not found any significant faults. So far, my tally is one pressure switch and one PCB, both fixed under warranty, and a dodgy pin in a connector which had everyone fooled for a while!
As far as I'm concerned, the C36 does what it says on the box and seems to suffer only from the same problems as other combis, notably a tendency to blockage of the secondary heat exchanger due to crud from the radiators. I always fit a Magnaclean - problem solved.
It also comes with weather compensation built-in - but not a lot of people know that (or indeed, what it is!).