If your sync speed drops to below 15Mbps, allowing for time to rule out a glitch (my words!) then your ISP (or CP as they call em now) may report a fault..
From "Conditions for Generic Ethernet Access Schedule 2B – GEA-FTTC Service"
Open reach PDF
If the downstream speed on a line falls below 15Mbit/s at any time after a suitable period is allowed for stabilisation following provision of the GEA-FTTC Service, the Communications Provider may report a fault.
This is obviously for those whose estimated speeds were above 15Mbps in the first place.
It also seems that on installation, if the engineer sees a sync of less than 15Mbps then he will revert back to the old ADSL service. (Unless the customer was told it would be slow and has knowingly ordered FTTC at <15Mbps).
I hope you are all following this