2 of the 4 on/off markers are loose and not bringing on the boiler and getting in the way of the 'clock' face as they block the way of the other 2 markers.

Gaynor Jenkins, November 2017

Probably not. It is simple enough to replace the timer; just slacken right off the 2 screws underneath, pull the lower edge forward and lift the unit away.

The new timer should just slot onto the old backplate and screws tightened. The only complication is to set a link or switch in the back, depending on whether your hot water is gravity circulated or fully pumped.

Users guide:

http://horstmann.securemeters.com/files/7414/6916/1817/user_guide_CentaurPlus_C27_Series_2_user_web1.pdf

Installation isnstructions

https://www.tlc-direct.co.uk/Technical/DataSheets/Horstmann/HOC21-C27_installer_Old.pdf

You can also take the opportunity for an all-electronic one without any mechanical bits:

This is a typical that should fit:

https://www.screwfix.com/p/hortsmann-centaurplus-c27-programmer-with-li-ion-battery-back-up/68934

HTH, November 2017