asif malik , 31 , sara kiran , 29 , and their four children last seen on april 7 .
left slough , headed to calais by ferry and then took train across europe .
they have three boys aged one , two and four , and a seven-year-old girl .
thames valley police ` extremely concerned for the safety of this family '
anyone with information should call thames valley police quoting reference 342 -lrb- 19/4 -rrb- .