Hi wondering if anyone could give me some advice as to the problem I've recently put the wrong fuel in ie high octane on my 02 reg fiesta and is now misfiring I think it may have blown a spark plug I'm wondering if anyone could confirm or give me any of cause or advice to what it is or to resolve

High octane isn't the wrong fuel.. It's simply higher octane

Petrol Cars in the UK require require Minimum 95 oct. High octane reduces the chance of premature ignition

The coil pack on the mk6 fiesta (I'm assuming it's a mk6?) is a very common problem. First of all id change that along with HT Leads & spark plugs.

This will also look great on the cars service history.

If it is indeed a mk6 then the washer jets leak and allow water to work its way onto the airbox and into the spark plug well. Mist type washer jets come with rubber seals to prevent this

Thanks for getting back I thought it was maybe plug or coil related as my cousins a mechanic but a second opinion is always good I've just serviced it myself new plugs as well so not sure to check if one has gone and yeah it is a mk6