HELP PLEASE!!! :-)

I recently purchased a 1999 VW Polo 1.4CL which has been running fine since I bought it six months ago, apart from the occasional engine cut-out at junctions from time to time (which I see from other postings on this web-site is not an uncommon fault in itself!)
I used the car on Friday night as usual and everything was normal, however when I came to start the car again on Saturday morning it just completely would not start at all! I'm completely baffled by this, especially as there had been no actual starting problems at all prior to this one. When I turn the key, all the dashboard lights flicker and it sounds like the starter motor is clicking and trying to start the car, but the engine just will not fire. We've tested the battery and know that it is fine and we've even replaced the starter motor with a working second-hand replacement from Ebay over the weekend but the car still will not start! Even bump-starting the car does not work. I can only guess that there is some sort of electrical fault, as this problem literally happened over night? On Friday night the car ran perfectly, on Saturdayt morning the car would not start at all and hasn't done since! Please would anyone who might have an idea what the problem might be, kindly get in touch with me.

Thank you very much, in advance! :-)

Kind regards,

Wendy D