I personally don't think any role is absolutely necessary for the running of a club (though there may be some roles essential for certain clubs, but not others). However, I think the following are most helpful: holiday sec, meets sec/social sec, treasurer, crumble officer, welfare officer. There may be some clubs for which other roles are more crucial - for example conservation sec if the club 'looks after' a particular cave system, membership sec for clubs with a bigger membership base/intake, hut warden for clubs with a hut, etc. Which is probably why it varies so much from club to club - circumstances vary, organisational structures vary, people available to fill roles varies, etc. I have known clubs before where Cheese Officer, Potato Chief, and Crumble Officer were important committee positions. There's a theme there!