We include VAT/tax in all our prices, and discount codes have always been set up so that a subtotal threshold requirement would be inclusive of any applicable tax.
It appears that since upgrading to 1.9.3 (from 18.104.22.168) Magento is no longer respecting the "Apply Discount On Prices" setting, as the same codes now seem to require the subtotal threshold to be met *excluding* VAT.
So for example, we have a code which allows £3 off on a £15 spend - most of our products include a VAT component. Before the upgrade, a basket with £15.50 would qualify - but now it does not, the spend needs to be £18.50ish (so that the pre-tax amount goes above £15).
Is this a known issue?
My issue was actually with Sweettooth Rewards - they have now supplied a patch which has fixed this
What Sweet Tooth patch do you mean? Is it the latest version? or just patch. Because I cannot see any patches on their site.