Related product links open in different template

Somehow - and I'm sure it's a setting in Hikashop somewhere, that if you click on one of the 'Related product' links it opens in the website default template rather than the base template the rest of the shop operates from.

Originally the URL changed from /merchandise/shop/product/.... to /hikashop-menu-for.... but when I disabled that in the 'Display --> Content menus' it altered the URL to /component/hikashop/product. The end result is the page opens in a different template.

Any ideas?

In the "related products" module of HikaShop that you can edit via the menu Display->Content modules, in the "menu" option, you can select the destination menu of the links of the related products. By default, it goes to a menu called "HikaShop menu for...etc" Since you disabled it, Joomla SEF system doesn't find it anymore and it doesn't have any menu anymore and thus the URL changes.
So you should simply set that option to your Shop menu item and that should be enough.

Easy! Followed your instructions and I've picked an existing menu - everything displays perfectly. Many thanks!