Defendants also referred me to Santley v Wilde

A mortgage is a conveyance of land or assignment of chattels as security for the payment of a debt or discharge of an obligation; a clog or fetter is a provision inconsistent with the idea of security (a repugnant condition) inserted to prevent redemption on payment/performance, which is void, but a collateral advantage (such as a share of profits) is not necessarily a clog and may be valid where the bargain is reasonable.