Sounds terrible. I had friends in a similar situation with a very damp flat.
A few questions just to clarify
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Is this a general problem or only during the recent very cold weather?
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Do you know what is causing the condensation - you mention poor insulation but could there be other causes such as damp from a leaking roof?
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What is the length of your current tenancy - eg month to month, year to year or a fixed period of so many months or years?
Where this is coming from is that if you have a tenancy for less than 7 years (this includes tenancies from month to month and year to year) your landlord is legally obliged to repair the structure and exterior plus the plumbing and heating apparatus, irrespective of what it says in your tenancy.
Therefore if the problem is due to some sort of disrepair your landlord is legally obliged to fix it.
You should also read your tenancy agreement to see if you have any additional rights.
(If your tenancy is for more than 7 years then your rights will only be as set out in the tenancy agreement)
I'm not sure what the position is if there is no actual disrepair and the insulation and/or heating system is simply inadequate. There are rules re fitness for habitation but off the top of my head I don't know what these cover although the website (mentioned below) suggests the local authority may also have powers to intervene.
Your options for forcing the landlord to take action are (a) to take him to court to enforce your private law rights above or (b) alternatively/in addition you may be able to persuade your local council's environmental health department to take an interest under statutory powers which they have to deal with damp rented accommodation if it is posing a risk to health.
This from the citizens advice bureau gives a useful overview of the law
www.adviceguide.org.uk/es/index/your_family/home_and_neighbourhood_index_ew/renting_a_home_index_ew/getting_repairs_done_while_renting.htm#The_landlords_and_tenants_responsibiliti
Is there anyone who could opine on the cause of the condensation for you?
A threat of legal action may be enough to make your landlord see sense but will have most impact if framed in appropriate legal terminology. It may therefore be worth consulting your local citizens advice bureau or similar for help with this.
Make sure you keep evidence of all your interactions with the landlord - put all complaints in writing or follow up verbal complaints with an email, and likewise make sure you have a record of the landlord's response even if only an email from you to him recording what he just said to you.
Hope this helps.