I know "abs are made in the kitchen" but I've recently lost some belly fat and, although I still have some left to lose, I think it would be beneficial if I started to tone my stomach and give it a helping hand.

At the minute I am doing the C25K and 15 mins of stationary bike 3 times a week and swimming once a week. I know that running and swimming help tone the stomach but I would also like to do some exercises specifically for my tummy. I know there are crunches and planks but I have no idea how many to do, how often to do them and where to start! Oh and bum exercise routines would be nice too!

Thanks in advance!

I would recommend youtubing some work out videos that focus on belly and butt toning. They're easy to follow and easy to do at home, and they explain and demonstrate how to do each exercise.

Planks and crunches are great abs exercises but yeah you have to have a low enough body fat to see your abs.

For planks, hold as long as you possibly can. At first that may be 10 seconds but if you keep at it it will go up I'd say once you know how long you can last than after a week or so push for a small bit more ex. from 10 seconds to 15. Rep wise, I'd say 3 to 5 reps.

For a great ab exercise we use a ball, Jazzercise its tough but it works. There are plenty of videos out there specifically for abs.

Planks yes, I wouldn't waste your time with crunches or balls or ab specific work outs.
Deadlifts, pull ups, chins. Those exercises work your entire body, specifically your core. You can waste your time rolling around on a ball or doing hundreds of crunches, but the real results are diet and full body exercises.