Another cause for large cracks can be applying too thickly.
Plaster shrinks as it sets. A thin coat shrinks by getting
thinner which doesn't notice, but a thick coat will shrink in
all directions, resulting in cracking.

Having hacked off my original work I agree that this is the most
likely reason for failure.

I took a photo so you can see the cracks before I removed it:

http://www.andy.hide.dsl.pipex.com/IMGP9763a.jpg
http://www.andy.hide.dsl.pipex.com/IMGP9763b.jpg
http://www.andy.hide.dsl.pipex.com/IMGP9763.JPG (Hi-Res)

The cracks ran right through the whole coat that I applied.

I have since re-applied a thinner coat (4mm instead of 10mm) which I
will then go over again. This has given a much improved result with no
cracks but using the same mix.

Any advice on building up in layers? Should the first coat be a
stronger mix than subsequent coats or can it all be of the same
strength ?