Irregular chiming Smiths Westminster Clock?

I have just bought a Smiths Westminster Clock which was sold as fulling working but is chiming irregularly.

It seems to be something to do with the quarter to chime as the first hour it will chime on the hour, the quarter and half but just clicks on the quarter. Then the following hour it will chime quarter to on the hour, then correct itself for the quarter past and half, but clicks again on the quarter to, then on the hour it is right again! If I tap it on the quarter too, it sometimes responds with the correct chime.

Help