Am just wondering if anyone has any advice on whether it is worth travelling into Narita from the airport with a connection of five hours. I'll be arriving at 7am and then departing again at midday. I'm guessing the timing is a little too tight, but would appreciate any advice.

I would get bored spending 5 hours airside in NRT, even in a lounge, unless I had work to do.

There really isn't much on offer airside in NRT, IMHO, including restaurants. The choice of eateries landside is much wider (this is an option).

For 5 hours, I would consider a quick jaunt to Narita town, presuming you already have your onward boarding pass. This should give you about 2 hours in town.