After hard acceleration then letting off the throttle, my Fiesta will often shudder a few times when coming down the rev range instead of smoothly coming down as expected. Is this normal, or is it a problem? Any tips would be appreciated, and whilst I'm here making a new topic I'd like to know how to know when to change spark plugs. I bought my car used so I have no idea if they were changed when they were meant to be.

Ford advise every 3 years or 37500 miles but if the engine is running well and no codes triggered happy to leave them longer currently over 5 years 21000 miles on current plugs and running like new.

Mine goes up and down the rev range smoothly so there could be an issue with yours 

if the rpm is going down smoothly and you feel shudder in the car body then I would check cv joints, the inner cv joints particularly for any play, I would check also engine and transmission mounts.

do you hear any metal to metal knock when you start to move from stop forward or reverse ? if so then I would definitely check the cv joints and the mounts.

this engine is very torquey , it has a habit of damaging the lower transmission mount , they call it ( the dog bone mount ) I have seen many cases like this, worth checking