gzt: I would firmly like to see some kind of rental housing warrant of fitness phased in. I don't know any details of what political parties are proposing, but I do think some kind of minimum would be a good start to avoid disruption.
Essentially a landlord/property investor provides a service and some kind of minimum standard for that service is required. As a secondary effect this would also mildly dampen sales of property unsuitable for renting under that standard and providing a slightly better entry point to first home buyers and renovators in that part of the market.
Basically I think that putting up with some inconvenience and rennovations as a first home buyer or renovator for capital gain is quite different from simply renting a below standard property where the renter is often paying the same rental price as other properties in the area anyway.
In the UK for many years local authorities have had powers of entry and inspection and can condemn dwellings as unfit for human habitation if they fail basic sanitation, heating and insulation standards. It originates from anti-slum legislation in Victorian times. Now of course it includes things like annual safety inspections for gas appliances and so on as well.
It's long overdue here judging from some of the horror stories people have related to me about places they've rented here.