I used some sort of 'quick drying solvent' stuff to clean the oil & grease off the hood. Once finished it was dry to the touch, but still looked like an un-shiny, matte finish.
The top photo shows it cleaned. The old pad wouldn't come off any better than shown, but the new one stuck onto this remaining material very well.
I then used Evostick - apply to both surfaces, leave to dry for 15 mins (till it feels dry), then push together. They bond on first contact.
Be careful - you can't move or remove it after they contact, so I lined the pad up first, marked a corresponding small rectangular area on the pad and hood, and stuck this first. Then I just worked towards the outside. It's stuck very well now.
better luck on your W123, Holson
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black