UPDATE: struggled for a week trying to mess about with it, went on to Amazon, ordered a USB drive, arrived 3 days later, plugged in, held down command key as it booted up, clicked on the USB drive and 15 mins later OS installed and sorted, no fuss at all!! Everyone go and buy this : https://www.amazon.co.uk/Bootable-Installation-install-upgrage-Macbook/dp/B077MJMVTC/ref=sr_1_1?dchild=1&keywords=high+sierra+usb+drive&qid=1588005905&sr=8-1 (check the OS will work for your iMac year) hopefully this helps everyone as it did me!