Thanks all. Looks like there was a misinterpretation of the pipe layout when the backboiler was replaced by the combi. Will look at blanking off that loop entirely, or maybe fitting a bypass valve.
Just put an isolation valve on that loop, you can use that to allow some flow if required, you shouldn't need a system bypass, if you have a one pipe system from the old back boiler blanking that pipe off ain't helping.
