I have never seen a 15mm zone valve, it will be 22mm with a 22x15mm reducer on each pipe end. This make of valves were never much good. I would replace it with a Honeywell 22mm zone valve, they use a ball on a lever arm to close off the inlet port (make sure that the arrow on the body is pointing in the right direction of flow. It Is considered good practice for the pump and motorised valves to have maintenance valves fitted both sides of the pump and after the zone valve /valves.
You sometimes don't have to change the compression nuts but from my experience the Honeywell nuts are a finer thread than most other valves. If you have to change the nuts often the old olives can be forced of the pipe or reducers ends using pump pliers between the back of the nut and the pipe end but don't go mad or you will damage the end, failing that cut off the old olives carefully with a junior hacksaw or an olive cutter.
Having just blown up your picture and it's bad to tell but I think they might have used 22x15mm reducing sets,if so I would remove them and just solder 2 22mmx15mm endfeed internal reducers on the ends of the pipes and put the olives onto these.
It's not to bad of a job if you can get a dish under it.
pete