@fBoyle-- That's actually a common problem with melatonin. I think that almost everyone who takes melatonin for more than a week will start building some tolerance to it. Using some valerian in addition may certainly be helpful. But have you tried valerian on its own before? Valerian on its own might actually work well enough for you. You might not even need the melatonin.
If melatonin is no longer working, I suggest trying valerian first. If you're not getting the results you want with valerian, you might want to add a low dose of melatonin and see how it works out. But like the article said, go slow. Start with very low doses and slowly increase them if necessary and never go above the daily recommended dose for either. Ask a doctor if you have concerns.