I heard a BBC radio prog a few days ago Rambler about a driver having his car insurance voided because some of his car's parts were non-manufacturer originals - thus his car had been modified and he'd not mentioned this when taking out insurance. He knew nothing about them when he'd bought the car second-hand from a dealer - who denied knowledge.
By the same way of thinking the changes to your headrest are a modification. This could create serious problems in the event of any insurance claim by you or a future owner. Take care.
I hear what you say , that's why I've purchased another one from ebay to replace mine when i sell.
And my current headrest is still the original, only mine is....comfortable for me ,no neck or back ache.
A head restraint is a head restraint, its in place doing its job. And its not an MOT failure.
Tbh...if a random person looked at my car now , you wouldn't tell the difference at a glance if you didn't know as the change is that discreet.
Thank you all for your concern though, really appreciated and taken aboard.