Lately, my dog has begun to wake me up during the night (around 3am to 5am) in order to go outside to relieve herself. Not only is this annoying as it disrupts my sleep but causes me grief that she is waiting for so long, busting to go to the toilet. :(

Is there any way I can prevent her needing to go at that time? apart from not letting her outside... that's just cruel...

and also, is there any chance she has incontinence? as she peed on me while she was dozing in my lap several weeks ago. (i don't think she was showing disrespect, because she loves me and is extremely obedient in most respects)

she is a 5 year old spayed who has had a litter or two. cavalier king charles spaniel.

Perhaps you could try picking her water bowl up after a certain time ... she might just be in a new habit of drinking right before she goes to bed. I've heard of parents with kids who wet the bed breaking them of it in that way. It might be worth a try and if you're not already, go out with her right before bed and make sure she's going potty. If neither of those things work, I'd suggest a trip to the vet. It could be a bladder or kidney infection or a weakened bladder from the litter(s) she's had. Good luck to you both!

It could be one of a number of things.

Spay incontinence is fairly common in dogs, this is basically when the muscles in their bladder are not as strong after spaying and they can't help it.
It can be fixed with surgery but it is very expensive and only selected vets perform these operations.

Another option is a urinary tract infection, same as with people these can be very painful and make her urinate frequently.

I would get her checked out at the vets to rule out any medical problem before moving onto behavioural.