I would imagine that there was some type of Landlord Tenant agreement/Lease which was signed. Try to look at that and perhaps you can tell what type of repair etc the Landlord has stated he will cover. I'd imagine any type of binding written contract like that could override the other UK laws.
For example, your landlord could have stated that he will do no repairs to any doors in his building, yes unlikely
. But if there was a law for the UK stating that all Landlords need to repair doors. I would imagine that him stating he would not repair doors on his lease agreement, and it being agreed to when the tenant signed the contract, would override that law and exempt him from having to fix any doors. (I'm sure there would be safety laws though that override this etc. etc. specially if the request is completly unfair, as something like this I imagine would be)
That's just a guess though, but I'd most certainly check out what papers were signed by your roomate, if she's the person on the main contract and see what was agreed to in terms of repairs.
I found this site, can't find out if it's free to join, like an online community or what...but you can give it a try
https://www.landlordlaw.co.uk/index.ihtml
You can always try searching for a forum that deals with this type of stuff, I'm sure there are plenty.