Funny taste/smell to water

It seems from first glance to be an issue with backfeed from somewhere, if you say that you dont have tanks and it is mains fed, then somewhere is supplying the water that keeps coming out the tap. A proccess of elimination, by disconnecting the incoming supply, see if that stops the flow, and try to trace where that water is coming from. As for the smell that is something that, by being different to each of us probably needs to be smelt to understand, but i too agree that the filling loop should not be connected but is unlikely to be the source of the fault.

What you first have to ask is whether you have a tank in the loft. And I assume it is cold water as hot water is rarely drunk ? You may have an open vented system. Sometimes, certainly in older properties feeds to certain taps may be fed from a tank in the loft. By Law kitchen taps should be mains water which has a significant pressure to it. If the tap is dribbling out at low pressure, it is possibly tank fed. Failing that you may have a defective mains isolator which is letting by, that is to say allowing some water past, or perhaps even a cross feed this can happen with mixer taps or shower taps where cold mains pressure escapes into the hot circuit due to a pressure imbalance. Only a visit would really answer these things. I do not believe that this is anything to do with the fill loop as this is for the heating system only and the primary heating circuit. Certainly nothing to do with gas, unless you can smell gas. Your hot water would be fed either from a separate insulated tank (hot) or a plastic cold water storage tank poss in the loft.