Seem's that you can do a standby after a duty but not before.
http--www.caa.co.uk-doc-33-FODCOM200229.pdf
Page 7 of 23, para 12.5.
Actually page 28 of 44 on the Pdf .
When any period of standby finishes, during which a callout has not occurred, at least 12 hours rest must follow prior to the next duty period. Similarly, following the end of contactable period or periods at least 10 hours must elapse prior to the next duty period.
Hope this helps.
