1996 mark XII - Service advice please.

Hi, My XII is 10 months into its IWC service warranty. It is losing one minute a week. Should I return it or simply set it a minute in advance every two weeks. I know the JLC movement can be temperamental. Thanks

Losing 8.5 seconds per day is not within tolerance and should be returned for regulation. It should be 0 to +6 seconds per day.

How long after the service did it start losing time as described?

I purchased this model two weeks ago with service history. I will do as you suggest and take it to the London store to be regulated. I hope it can be done in house rather than being shipped back to Switzerland.