what should I do ?

I've just been cleaning the buns out and I heard a meow so I think the new neighbours have an outdoor cat

What should I do to make my buns extra safe

My neighbours cats have always been too scared to get close enough and do any harm... We have our own cat which helps as she will chase them off, but she is quite scared of our bunnies so always keeps her distance...

What set up do you have at the moment? xx

Your runs should be secure on all sides including the roof and floor. Fit padlock bolts to all doors. Make sure all mesh is secure and strong enough. Reinforce any weak wood.
Foxes are probably a bigger problem, but you need to protect from everything

Agree with Stator - a fox is far stronger and has a far greater instinct and incentive to get to your buns, so if you've already made sure that your accommodation is as safe as possible from foxes, then the cat shouldn't be a problem