You are correct that they could be marking because of each other. They are all fixed, which is good. Have they all been properly bonded with each other? I have no experience in bonding, but from what I’ve read, I think the marking of each other’s territory lessens/stops once they’ve established the proper rapport with one another and do not feel the need to mark.

— And generally speaking, no. It’s very dangerous to let pet rabbits run outdoors. There’s constant predators, loud noises, potential for parasites, infections, poisonous chemicals, etc…..

You can make a safe run for rabbits outside, but the temperature difference between outdoors and indoors should preferably not be more than 10 degrees Celcius up or down.

A safe run has netting on all sides, 1/2 inch by 1/2 inch welded netting is the best. The absolute best thing is to dig out an area and put down netting and then fill up with earth, but if you can’t do that, netting on top of grass is fine. Girl buns especially likes to dig and they can dig quite impressive tunnels in a very short time. Predators can also dig in, but most predators are active at night and that won’t be a problem if the buns sleep indoors. There must be a roof on the run because otherwise birds of prey can swoop down on the rabbits and kill them.

Since you are in England, the buns need to be vaccinated against Myxo, RVHD1 and RVHD2.

ETA: Our member Vienna Blue In France lets her buns out during the day. She’s in the north of France (=similar climate as England). You can PM her and ask about her arrangement. 

As long as you adhere somewhat to the 10 degrees C rule, I think they’ll be fine. Make sure theres some type of hidey-box with bedding in it so if they do feel cold or if it starts to rain, they can hop into the box and keep warm. The box should be elevated off the ground a little bit and there can be straw in it. Rabbits are good at keeping warm, but if they get wet or there’s a very cold wind that they can’t get away from, they will be cold.