My mother has Now TV but recently the box has become very noisy and has started to overheat to such an extent that it needs to be unplugged for several hours to cool down. The box is in the same place it has been for several years. Trying to report a fault is a nightmare.
There are a number of communication methods to reach NowTV on this link below.
I am assuming your Mother is using the NowTV Freeview Smart box (blue NowTV logo on top of the box casing) which i believe is the only NowTV box that has an inbuilt cooling fan.
Apparently this is quite common in boxes that are a few years old. They advise turning it off for 15-20 minutes to let it cool down - thats really practical when you have to do it every hour or so.
Obviously a fault with the boxes but they don't seem to want to do anything about.
Whenever I have a small electrical device switched on for a long time I use a desk fan to constantly blow air on it, that way the device is kept cool.
The fan itself can be sized between 4 to 12 inches, with 8 inches being a good compromise (sorry, don't know what these measurements are in metric).
Hope this helps.
I just found this post after googling "overheating NowTv box" perhaps they're not made to withstand our British summers! I've turned mine off now for a bit... It's only been on a couple of hours though. Could've fried an egg on it!