The Avenger II Seawolf is automatic, so no battery. (I guess you've worn in constantly for the last 4 years or else you'd have noticed that it stops after a few days of non-wear).
-5/7 seconds is only just outside of COSC specs (+6/-4) : what was it running at before? It's coming up to a service window after 4 years, so if the time-keeping bothers you then a service certainly won't hurt.
Service prices are here (although you'll need to change it to Euros) -
https://www.breitling.com/gb-en/service/service-prices/