Got my 456 built up, but the 350mm layback seatpost I am using is slightly too short to get a really efficient pedalling technique for fast XC rides (18" frame, 5'10.5" tall, 33" inside leg).

I am considering getting a 400mm seatpost but don't know whether to go for inline or layback. I get the impression that the bike was designed for inline seatposts, but I feel like I am at the upper height limit for the frame. Of course, If I do raise the saddle it will move rearward slightly. The other option is a 10mm layback post I have seen.

Anyone got any opinions? Especially those with similar dimensions to me.

I get the impression that the bike was designed for inline seatposts

Bikes aren't designed around inline or layback posts (unless it's custom I guess), that's down to rider fit / preference.

I'm 5ft 11.5", 33 and a bit inside leg, ride an 18" 456 and use a 400mm inline post (on-one twelfty in fact).

BUT every one likes different set ups. Might be worth experimenting.

OK fair enough, I think I misinterpreted some text on their site:

Sliding your saddle back will make you do wheelies

Difficult for me to decide what to do, I don't know whether a 400mm inline would be right or not, because I haven't ridden with a 400mm inline!

See how much front and back adjustment you have with your current saddle, if you can go back another 20mm you don't need a layback. Raising the post an extra 20mm or so won't affect that by more than a couple of mm.

I run mine with a fair bit of layback (5'11" on a 18), and an inline would be nowhere near the right position but it does depend on how much post you have showing. The more you have the more likely an inline will be suitable. Just buy a cheap post and experiment…

I have a 16" inbred and am about 5'9. Use a 400mm inline thompson and the fit is pretty good. The inbred is a pretty low frame and you can find yourself using a lot of the seatpost, which puts the saddle back – if its too far back then an inline post can sort this out. A layback would put it even further back, so probabably not a good idea. But really depends on you…

I thought 456s had particularly long chainstays though to aid climbing, so a layback might not be as much of an issue as it could be on a bike with 'normal' length chainstays?

Hmm difficult. I will be having the seatpost out quite far, since it will be out further than a 350mm at max extension, which might suggest an inline.

At the moment I have the saddle quite far back, but this is mainly because it's the only way I can increase my leg extension without going past the 'minimum insertion' mark.

inline with pikes, thomson layback with 100mm rebas.

I'm 6ft, 34.5" inside leg and ride an 18" with an 80mm stem. I find the inline helps on climbs when running longer forks. If I had to buy a new post I'd get a normal layback and just shuffle the saddle back and forth. There should be enough adjustment room for you to experiment already, if there isn't then you already know the answer about which flavour to try!

If it helps here is the picture of a set-up I was happy with. The only problem was not getting enough leg extension. I am tempted to go for an in-line based on this photo.

Looking at that pic I'd say you were on the right lines, but the pic disagrees with you saying you had the saddle quite far back. If the pic is how you like it, get the inline.

Since that pic I did move the saddle back, but only because I needed more distance between my bum and the cranks, not because I wanted to sit further back, if that makes sense. Probably not!