Need to replace some fencing in the back garden. Whats the best way of stopping the posts falling over? Concrete in or those metal spikes?

Concrete.
Haven't touched my fence for about 12 years since we did concrete posts, plus concrete panel (about 30cm high) to keep the woodern panel off the ground.

Concrete or postcrete. Form the concrete about 2-3 inches above ground level and taper it away from the post. This will help to stop it rotting so fast and keep it much drier

Concrete. Believe me. If you use metal spikes alone you will be buying concrete after the first high wind and doing the job all over again.

Postcrete, but make sure you use plenty of water.

Postcrete flame wars: http://www.diynot.com/forums/viewtopic.php?t=68920

I think you're getting the general idea, but concrete.

i had lots of old tree roots and stumps and digging holes deep enough for concreting in was tricky so I dug the deepest hole I could and used metal spikes and drove them through the roots, but I then concreted up the shallow hole as well.
Solid as now
Important with spikes to secure the post to the spike (coachbolts or something), dont just rely on a friction fit into the top

Concrete it is then…

Thanks.

I'm sure concrete is best but the posts with spikes are salvaged for use in my chicken pen look like they'd be pretty immoveable once embedded properly – how to bang them in though? To use mine I cut most of the spike off.

Castle postcrete is good, hanson is rubbish

I use lengths of 2" x 5mm thick steel angle. Drill them for 10mm bolts to the wooden posts. Slap on some Waxoyl, conrete them in. They'll last ten times longer than metposts and very cheap, especially if you find a salvage yard for the steel.

I concreted a metal spike in…

+1 for post crete. Being a fencing contractor we use post crete on some pretty steep hills with plenty of water and we put around 1 ton of pressure onto the posts they don't move

I concreted a metal spike in..

I,ve used the specific ones for concreting in for fence posts and a large raised deck. The reason fencing contractors just use concrete or postmix is that it's cheaper and they really don't give a damn if your posts have rotted out after 5 years.

Main reason postcrete is used is because it sets in 20 mins. It actually costs about 3 times as much as concrete.

And it works!