traditional yale nightlatch is rubbish. It will usually prevent the door blowing open in a strong wind, but your granny could kick it open.
You need a British Standard thief-resisting mortice deadlock
(if you insist, you can get a BS nightlatch at five times the cost but it will not be as good)
Take the key with you when you leave so that an intruder has to carry her booty away through the window or back door. This will inconvenience her.
Unless you are good at carpentry (it needs a chisel) get a local carpenter or joiner to fit it.
If you have glass in the door I recommend replacing it with 7mm laminated, which is available in several obscured patterns, retained with adhesive glazing tape as well as small screws on the bead (not putty, please).
You current problem is that the person breaking in has learned that the house is empty and nobody will mind if she breaks in. She may be planning to come back later to take what she has seen. Especially, you should not leave power tools unattended.
Once you have fixed the front door, you need to fix the back door and the downstairs windows. You can fit bolts but unless you use metal locks and take the key with you, admittance can be gained by breaking glass, and then they can open up.
You could fit a burglar alarm. For a low-price DIY fit, you could get a Yale HSA6400 from Screwfix or Ironmongery Direct, usually on sale down to about £160. As well as a siren, it can dial out on your landline when/if you have one. You need a drill and a screwdriver to fit it. If you have no electricity supply you can get a totally battery operated one for less, but it is not as good. Your neighbours will ignore the siren but it may make the intruder feel nervous.
A wooden door is generally stronger than a plastic door, and a lever lock is much harder to overcome than a cylinder lock.