In article ,
(Andy Hide) writes:
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 didn't realise you meant the scratch coat. I don't think
this matters much -- the cracks give some extra key to the
finish coat. May be an issue if the Sand/Cement/Lime is being
used for its waterproofing properties, as the finish coat in
the cracks would bridge the waterproof layer.

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 ?

I think the same mix. You should wait at least 24 hours to recoat
sand/cement (with plaster or more sand/cement), but not long
enough for it to dry out (around 48 hours for 10mm IME). If it
does dry out, you'll have to PVA it. In this weather, you'll
probably need to dampen surface a bit anyway.

Can't tell the scale of your photos, but I usually do more devil
floating (scoring of the surface) than I think you have done.

--
Andrew Gabriel