I have previously used a mix of 5:1 Sand/Cement to patch up blown
areas of plaster in my c1910 house. I have then gone over this with
multi-finish. Some of these patches have developed hairline cracks and
it was suggested that adding lime to the mix would help allow for any
slight movement of the house. It was built with lime mortar.

I have now changed to a 6:1:1 Sand/Cement/Lime with the sand being
Wickes (red bag) "for use in bricklaying and internal rendering". Is
this the same as plastering sand ? Would plastering sand be better in
this case ?

Having left the new mix to dry out overnight I come back this morning
to find large cracks have appeared throughout as it has dried out.
Things are so bad I think I am going to have to start again. Cracks
are worse at the edges where the wall meets the door frame (weakest
point ?).

A few thoughts on what I think may be going wrong:

1) I'm using hydrated lime which is white and very fine powder. Did I
get the right stuff from the BM ?
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 ?
3) Is the 6:1:1 mix to weak ?
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.
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.

Any thoughts on this would be very much appreciated.

Andy.