I appreciate it is not actually the sensor but the glass in front of it.
I have had the camera about a year and three months ago experienced problems with what I thought was dust but it truned out to be oil. I had failed to clean the 'dust' and ended up at having it professionally cleaned.
After three months I have the problem back again.
It was suggested to me that factory over enthusiastic lubrication in the shutter area could be the cause.
Is this likely? If so what do I do to prevent it recurring after I have paid another pro-cleaning cost.
It is most frustrating.