Best way to repair cracked render ?

There is a small area of render, on the outside of a ground floor timber framed extension of my house, which every year seems to suffer frost damage. This year it was particularly bad, one crack is about 800mm long and 3-5mm wide with a couple of other ones nearby not looking good either.
I'm getting fed up of patching it all the time. Obviously water ingress is the root cause here, but I can't find any damp on the inside, all the rainwater goods are in good shape and the roof isn't leaking, either.
What would be a good way to repair this permanently, short of demolishing the extension and rebuilding with bricks ;-) ?
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Why is water ingress the root cause? It could equally well be settlement of the new addition, subsidence or heave.
Richard.
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