If you're wanting customers to be able to customize their selection of items within a product you can use a product customizer app. There are a number of apps that have this function and you can find a list here.
Download a few of the apps and use their trials to determine which one would work best for your store as they'll all be slightly different in look and function.
Thanks for the reply - lots of options to choose from and all seem to have LOTS of functionality.
In reality, far more functionality than I actually require as all I really want a customer to do is select the beer they want to create their own 'pick & mix' pack of 12 where they could have varying quantities of each beer to make up their 12.
Any suggestions which would be best to achieve this at a reasonable monthly fee as my volumes aren't huge at the moment?
The app Infinite Options does have a free plan that allows you to put a customization on up to three different products, so it might work for you!