How do I stop our 8 month old Cavalier King Charles Spaniel soiling in her crate at night

Walk him at least 3 times a day, including before your bedtime.
Make sure the crate isn't too small and closed feeling. A wire pen type crate might be better for the house, and keep it near your bed for a while. He might be lonely and nervous if not getting enough time with you.
Make sure he gets plenty of play and patting and holding before bedtime, so that he is worn out and happy.
Don't withhold water, but you can feed earlier.
Praise him effusively when he goes outside.

Actually a smaller crate can offer more comfort. Dogs are den animals, and a den in the wild is usually only big enough for them to stand, turn around and lay down, and on the contrary, if the OP's dog is suffering from separation anxiety, a blanket (creating artificial darkness) over top of the crate can create a safe, comfortable place.

I do agree with keeping the crate in the same room as you. As well as more exercise before bed will increase her doing her business outdoors instead of in the crate. Also you might want to try feeding her dinner a little earlier in the evening. My dogs get fed a 5pm, out side for walks by 6ish and out side for their last potty break by 10 and then in bed. This is just a schedule that works for me, it may be trial and error before you find one that works for you.

Good points here. Tell us what time you feed her. Dogs most often need to be outside an hour after eating. How often do you feed her and are there accidents during the day? Did this just start or has she aways had this issue?