So I took the buns to the vets today just for a check up, and the vet says because stitch is tiny and sometimes has runny poos, I need to have increased amounts of fibre in his diet, anyone got any ideas what else I can give besides pellets, hay straw etc ive been told to leave off the veggies for a while, any help would be fab! 

Good advice about checking the fiber content of your pellets and hay.   What type of hay do you have. Most hays are fine with fiber content, but alfalfa is one that can be too rich for adults.  How old is your bunny?   Also, if you decide to introduce greens back in again, I dandelion greens and parsley are also high in fiber. Kale is okay, but I am not sure I would introduce a green from the cabbage family in first since your bunny is having digestive issues. 

What greens do they normally get?

My vet recommends cutting down on the sugary treat foods (so less apples and carrots, and only about a thumb size TOTAL of treat per day). I believe broccoli is not good for rabbits either, but I would recommend cutting them all out for now. If you feed them pellets I would cut that to about 1/8 cup per day each. Make sure their diet is pretty much 99% hay, and timothy hay is best.

When Stitch's poos start looking more normal again, you can try to slowly reintroduce vegetables just like you would a teenage rabbit (1/2 oz per day, new veg every 3 days, monitor the poo). If they start looking runnier then you'll know which veg is causing him trouble.