Scrap that it's stopped working again. Worked this morning. Came back this afternoon and nothing. Ugh
Ok I think I've finally found the issue. If I use a private browsing window (Safari) the promo codes work fine, and if I go into Safari settings and clear all website data then back to the shop and use a code it works!.. Which has me stumped to be honest. Are form submits effected at all by things like cookies? Does Perch store any data in the browser that would effect the promo codes?
I'm not sure why it worked earlier now, unless changing the PHP version and creating a new promo code had some effect on something stored by the browser when revisiting the site. Again why I didn't run into this issue on the local copy of the site using the same browser I don't know. Honestly feel like binning the whole thing right now. It's always the last 1% of a site which ends up being the most time consuming!