how can i fix my ford transit 2.5 diesel altnernator problem?

i had a second hand alternator fitted 3 months ago. about a week ago the fuel pump was leaking and therefore i had this replaced with a second hand one.

i had driven the van for almost 200 miles during which period it had been switched off and on a couple of times for stoppages. whilst on the motorway the van cut out and i seemed to have no battery power to start the van. i used leads to start it up yesterday and ran it for around 25 miles before it cut out again. can anyone help!!