Ok, all you peeps out there who have not done your Water Regs, the double check valve is required as most plumbers know to protect (along with the removable connection) against back flow of the dirty category 3 domestic heating water into the nice clean mains water which is category 1, therefore the protection device has to be fitted to the mains cold water otherwise dirt can enter.
This means that those cheap filling loops which only have one isolation valve can only be used if another valve is supply & installed, so you might as well buy the proper ones, doing this also has the benefit of providing a 15mm drain down, flushing & injection point, normally below the boiler & if the check valve fails it can easily be replaced without the need to drain the heating.
Please remove the filling loops after you have finished filling it is a requirement (sorry a pet hate), hope this clears up any confusion.
