How to start my van?

I have had a new battery fitted in my vito deisel van, the correct battery would not fit so i was given a smaller battery with the same output. the very next morning the van won't start! What is confusing is that the van isn't even trying. As i turn the key just 2 clicks there is a quiet whirring sound from behind the dash on the passenger side, until i turn the key the final click to start the engine and then nothing. I have checked the power output from the battery ( 12.5 v) the lights are working and the battery seems to be ok but it is some coincidence that this problem has accured after a new battery has been fitted. I have tried all that I can think of, jump leads and pushing the van in second. does anyone have any idea what the problem could be??