We are at present trying to mate our two dogs, trouble is just before he enters her, he jumps off, and the moment is ruined if you know what i mean lol!
Do we need to stop him from getting off at the right moment, or will they do it by themselves when they are ready!????

What breed are they? How old are they? Are both dog and bitch maidens? That often causes problems. Which day of her season is the bitch at? Has the dog been scolded for mounting in the past - that may have inhibited him too much. Matings can be very complicated, and you need at least one person in attendance who is experienced in handling matings - because it can be literally 'hands-on'.

It is not that unusual for dog to refuse to mate a house/kennel mate.