Hi,
The option "partner key" of the configuration of HikaShop will define the variable name in the URL. The value of the variable will be the ID of the user which is an affiliate.
Then, you can use any URL of your website as an affiliate URL by adding ?aff=ID at the end.
So, if you affiliate has the user id 91, and you want your users to land on the link you posted, he should use the URL
www.festivalofsins.co.uk/store.html?aff=91
What you can do is have a list of possible URLs like we have on our website if you go to Your account->Affiliate program
That way, the parmeter of the affiliate is added automatically to the URLs by the system. You can configure the URLs in the menu Affiliate->Banners
You can create such menu in joomla by choosing the type HikaShop Affiliate program