Define what you mean by 'breathe'. In the modern world it makes no sense.
Old buildings made with lime products were meant to breathe. That is, the lime allows the absorption and evaporation of moisture (unlike cement which is impervious). This is how an old building like mine stays dry without a DPC of any sort. I have demonstrated this in the last 18 months from starting with a cold wet heap which is now dry and warm. See my blog
http://houseintheenchantedforest.blogspot.co.uk/ and look at the 2010/2011 entries labelled "dampness" and "lime mortar" etc.
The walls are 600mm thick and appear to be suffering from rising damp, mainly affects the plaster about 300mm up from the floor.
I am not convinced on an injected DPC and would rather 'tank' it from the inside, although all tanking products boast about holding 7bar of water pressure etc.
However i dont have any pressure as such as its rising damp, so i was thinking of sand/cement render with waterproofer in, and then plaster over the top.
If you want your cottage to be free of damp you must treat it correctly.
1. Do nothing in a hurry that you might regret later; think before doing anything permanent!
2. Do NOT inject a DPC - at best it's a waste of money; at worst you will make the problem worse.
3. Tanking does NOT help; all it does is to drive the moisture elsewhere and you just chase the problem around endlessly.
4. Do NOT use cement, gypsum or any modern waterproofing at all - they will only add to your problems.
5. An old house was designed to work in a completely different way to a modern (cement etc) house. Basically, lime and cement do not mix!
As a priority you must tackle any causes of water ingress such as faulty roof covering, flashings, gutters, downpipes, drains and high ground along walls. My house had all of these faults! Any dead plaster should be removed toether with the nasty cement you will doubtless find on various walls and floors.
When you've done those, let it dry out for months - mine took about four months to start feeling better, and a year to dry completely. Then there's the cement pointing to be considered - I'm now at that stage. This just holds water into the wall because it cracks and the water gets behind but doesn't get out.
I could go on - there's still a disappointing amount of poor understanding about old houses, partly due to those TV programmes where people are encouraged to throw modern materials at an old house. I suggest you google "period property" and "dampness" and look carefully at what you get before you decide on anything.