it was suggested that adding lime to the mix would help allow for any
slight movement of the house.

Adding lime to a cement mix wont do that cos you get a cement set, not
a lime set (the cement inhibits the lime set) What it will do is make
the mix fluffier and easier to work

1) I'm using hydrated lime which is white and very fine powder. Did I
get the right stuff from the BM ?

Yes

2) At first I thought this was the problem of the wall sucking out the
water from the render too quickly thus it drying and cracking - but
having throughly soaked the wall to the point where it wouldn't suck
in more water I don't think this is the problem. Perhaps it's too wet
now ?

Cracks are worse at the edges where the wall meets the door frame

My immediate reaction to the 'cracks are worse ...' is that you have
not killed the suction enough where dry materials meet the mortar. But
then you say no. Could you try wetting the wall (and especially the
edges) well then allow time for that lot of water to soak in before
wetting again - say three sprayings in 20 minutes? Too wet a wall does
not give that problem, the plaster just won't stick to it.

4) Have I overmixed ? I'm mixing up in a large bucket with a whisk
attached to a drill. I guess you would normally use a cement mixer but
as I am dealing with relatively small amounts I thought this would be
OK. I'm adding the water first followed by cement, lime and then sand.
Adding the lime certainly makes the mix more fluffy and easy to work
with. Perhaps it's now too "bouncy" and full of air and as it dries
out it's shrinking ? Mixing with a whisk is probably a tad more
violent than a cement mixer.

I'd never come across this suggestion until a few weeks ago when it
came up in a thread here. Written on someones bag of readymix were the
words 'do not overmix'. So it is possible it appears

5) I've been applying to about 10mm thickness. I have found the
thicker I apply it the worse the cracks are but never had any problems
with this thickness before adding the lime.

That should be OK. No thicker though

Anna
~~ Anna Kettle, Suffolk, England
|""""| ~ Lime plaster repairs
/ ^^ \ // Freehand modelling in lime: overmantels, pargeting etc
|____| www.kettlenet.co.uk 01359 230642