I was thinking that if we do need a better aerial for receiving Freeview TV broadcasts, is there an ideal position where we can put it? I was thinking of putting it at the rear of the property like in the picture. Is this OK? It's a backup plan in case the local engineer wants an arm and a leg to fit a new aerial. It saves us climbing onto the roof which is a major risk if you're not used to it.
There are many factors that affect signal strength, some are in your control like antenna placement, but most are not (other buildings/structures, topography, location of broadcast, atmospheric conditions, etc.).
If you are trying to find the "best" placement then you need to do a site survey. The crude version of this is using a long temporary cable, moving the antenna around to different locations and viewing the signal strength from your receiver ("box") or TV. Professionals will use instruments like signal meters of varying sophistication.
One nice attribute of digital OTA is that being binary (ones and zeros), picture quality isn't affected - it either works or doesn't work (for the most part). If you placed the antenna at your desired location and could get say 75% signal on most channels, then you could very likely use that location even if there were better ones.
The key factors are height and the direction it's pointed in. Whether it's on the top of your house, or in the back yard isn't as much of an issue. Use whichever works best.