Halfords will sort you a free test on battery, I had the alternator and starter checked too to be sure.

Heard that the HT leads should have some white grease on there as an insulator on both ends, seems to be a common trend amongst the Zetec and -E models with rough running. (Premex Dielectric, but spray silicone should do the same to prevent water, dirt and dust ingress)

*fingers crossed* everything is sorted RE: misfire. Will keep this updated if it comes back & I figure it out.