2015 Focus 'Engine Service Now' warning

Just wondering if anyone has any advice. Our Ford Focus has had this 'Engine Service Now' warning come up on start up for years now, with no noticeable difference in performance ( as in, it's not in limp mode) - the reason it's been like it for years is because when it first happened, we thought it was just a service interval warning and since it was not long due a service and there was no apparent issue with performance nor any engine light on, we just waited to get it serviced - once serviced with the local garage, they said they were unable to turn the light off and that it would probably just need to go to ford for a service to turn it off. Well since it was an order of magnitude more expensive to get it done at ford, we just continued to get it serviced every year with the local garage and put up with the light. 

However, I'm now looking into it since we are selling the vehicle soon and now know that it's not a service interval light. The local garage ran a diagnostic on it, but cannot find any faults. We're at a loss now because it's a month-long wait for any of our local ford service centers to do a diagnostic and they all want nearly £200 just to do the diagnostic!!