Mantle Clock strikes out of sequence?

I have inherited a mantle clock which strikes 15. 30, and hourly chimes, it has 3 key holes in the front and was made by the alexander clock company and the internal works are stamped SMITHS.

The clock runs fine and keeps time , however the chimes are all over the place, the hour chime is 5 hrs behind time and all the chimes are striking 5 to ten mins early ....... is there a easy fix lol