They will be flat because I want to make a seat out of them
In fact guys do I need to fix them with a long steal rod if
A. they are laid width ways not height
B. I have 250mm timber fixings
C The first one is cemented in
I've used 3" (75mm) posts concreted in to attach fencing timber to for my raised beds so I'd think that's a good option. You'll need something fairly long and sturdy in the way of bolts to fix from the back of the posts through to the sleepers, but I reckon that would be ok. Laid that way they'd be quite stable anyway. Put the posts in to a good depth as well and don't skimp on the quantity. I presume you meant 25mm timber fixings, not 250mm, but I'd go with 75mm and get them down to at least a 300mm/450mm depth. You can countersink the bolt heads on the inside of the posts so that you don't catch your hands on them when you're planting or generally working in the bed.
Others may think differently but I'd be fairly confident with that, cd
They are 250mm (25cm) long screws. I think I will drive a really long spike through them on either side as well.