How to replace the timer on a greenstar 28i junior?

You do have to remove the cover. See the installation guide.

All these links are what YOU should have been able to find with YOUR search:

https://www.worcester-bosch.co.uk/support/document/download/release/8716115166/12640

Is this an MT10 timer?

http://www.diynot.com/diy/threads/mt10-timer-failed-on-worcester-greenstar-28i-junior.338440/

https://www.worcester-bosch.co.uk/professional/support/documents/worcester-mt10-installation-instructions

Anon
February 2017