Keston C36 Combi boiler - Error E37
Knowing nothing about your particular boiler, if you are adding water to the loop and water flows out the exhaust pipe, it sounds like your heat exchanger is leaking, as that's where a connection between the loop water and exhaust would be. 

Whether that's a simple part replacement or "new boiler time" will depend on the design of the boiler and price of replacement parts.

When it's fixed, I would personally want to have a pressure or pressure/temperature gauge (analog/non-electric) installed on the loop for my own information/peace of mind/ability to quickly check. Rarely used, but nice to have when there are problems.