sargent seat and rear cowl

Just got a 2-part sargent seat last week. Thinking of fitting the rear-cowl in place of the rear seat, but if I do, I can't see what there is to prevent the front seat sliding backwards.... Is this a potential problem or more just a worry? I'm also having to modify the rear seat bracket on the sargent this morning - the seat latch won't engage into it... doh.