Valliant Eco tec 28?

I'm not at all sure what you mean by "I can't turn the screw...as it looks like it has threaded"

The installation manual is here:

https://www.vaillant.co.uk/downloads/ecotec-pro-instn-maint-261423.pdf

Section 10.5 says: The system is filled via a customer-installed filling connection, which must be attached in a suitable position in the heating circuit. This connection must be removed again after the filling process is complete. If the local Water Authority regulations do not allow temporary connection a sealed system filler pump with break tank must be used.
The heating system will not be filled automatically from the domestic hot water side. (Alternative methods of filling sealed systems are given in BS 5449)

So; are you talking about the filling loop when you say "as it looks like it has threaded"? Has the filling loop been left in place?

Consider getting an engineer in to look at it and talk you through what to d next time.

XXX
February 2017