Ok, managed to get this one working as Ibobriakov posted, the only difference was that I seemed to state in the login call &actionKey. The call on my login page [[~28]] is as follows:
[[!Login? &tplType=`modChunk` &loginTpl=`myLogin` &logoutTpl=`myLogout` &loginResourceId=`13` &logoutResourceId=`18` &actionKey=`service`]]
Whilst the &actionKey defaults to service, it didn’t seem to work unless I actually set it.
Then I put the call for the logout as follows:
<a href="[[~28]]&service=logout" >Sign out</a>
Hope this helps another perplexed individual!
Oh yes, and when you’ve made these changes its worth clearing the cache and flushing permissions too...