Solved it!. If you open the info line it displays the Start, Value and Type. The Value field shows the Current Tempo just like the Toolbar. If a single event is selected the info line fields are shown in white. If multiple events are selected the values are shown in orange.
Double clicking on the Value field allows it to be edited. Select multiple events and then edit the Value. In this case ALL time events move up relatively.
So that was simple then. Undocumented (and I spent some time searching) Documentation only ever refers to changing single events.