So I'm after a new seat post to replace my borrowed one, and I don't know whether or not I would be better off with an inline or laid back post? Can anyone help?
Thanks
Ben

depends on the bikes sizing for you.

whats the borrowed one?
how does that feel in relation to bike you've ridden before?
is it on the KM?

^ Agreed, it can depend on sizing. For most of us on the right-size bike we could use either but one will feel+fit better. As a very general rule and only in my opinions from trial and error, I think you're better off with the saddle a bit further back than further forward on a bike that's going to do long distances. It helps balance your weight so that your core muscles support you more than your hands/arms would do if you're tipped further forward. Bring the bar back/up to account for that if needed. One possible drawback of this is for those with short hamstrings as it can create more tension there due to hip/leg angles, but for most it's not an issue just something to be aware of.
For an MTB that does a lot of techy climbing and descents, particularly with a bigger fork, shifting forwards can be a good thing and you won't be sat down as often anyway. hth somehow.

It would depend what you're trying to achieve/ change from the current post you're using. I'd use a layback post if I needed to adjust/maintain the overall cockpit length, but where reach adjustments at the bars/ stem end are going to negatively influence the handling of the bike.

How far back on the rails is your current saddle?

If it's all the way back look into a layback post, but if it's middling stay with what you've got (this may change the more you ride and fiddle about with things)

The borrowed post is a cheapy felt laid back one, it feels fine, so I should probably just stick with lay back, I asked just thinking if there was any advantage to an inline post? I certainly prefer the look of an inline post, but that's irelivent

If your position on the bike feels okay and your saddle's not pushed really far forwards then as you say, stick with a layback ... how much layback's another matter!