Technically, it will be the noseweight limit, which almost certainly will be the manual. But in reality, I'd say you can go a bit higher as 75kg of bikes bouncing up and down on the back will still be less than the same noseweight of a 1000kg caravan bouncing up and down. Can't see anyone trying to weigh everything unless you are blatantly taking the P with four DH bikes crammed on.
What is more important is rear axle loading, easy to go OTT once you have a bit of camping gear. Add the bikes and its all to easy to be bouncing off the bump stops…
Edit: If the car was made after 1998, it has to have a type approved towbar which will display a plate, on this will be an 'S' value, which is the max noseweight for the car. Its a guide rather than written in stone as the towbar will be usually be suitable for all versions of your car, often the larger engine models can tow higher weights and therefore could have a higher noseweight.