Thanks mate, that's now more making clear to me .#janis I used these hidden fixings off eBay. I just used the original hardboard. I stuck the carpet and then twisted these in from behind after.
I have since found them cheaper, I’ll post the link, but took a week to arrive, whereas the dearer ones arrived next day, which is what I needed.
https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/362506022083