i suspect that you did cut power to the oven when you replaced the element. therefore the oven needs to be reset as new. there are quite a few pages of instructions on setting the timer so be prepared if you are going to to proceed with this "little" project.
I was able to locate your manual online here:http://kitchen.manualsonline.com/manuals/mfg/zanussi/zuq_875.html?idRes=625295
I suggest unplugging it for a few minutes...then plugging it back in. Hopefully if the electrical is good you will see the oven's timer flashing. then you can proceed with the instructions from page 7.
Otherwise, you may have a blown a short and should get a licensed electrician to look at it.
Another option would be to contact the manufacturer:email@example.com
Or, find the bloke that installed it and invite him over for dinner and maybe he'll notice it's not working and will fix it for free! ;-)
Good luck mate!