Petition Maintainance of flats - landlord's obligations

If you suffer from a defect in your lfat, let's say the heating doesn't work in the winter, it seems that you can deduct a bit from the rent but when the landlord has repaired the faulty item after e.g. 4 months you have to pay him the full rent even for the past 4 months although the heating was off all the time. So I was told by the Wimbledon citizen advice bureau. This does not really encourage landlords to speed up repairs. I would have a proposal here: in some countries you have tables with percentages you can deduct from the rent: e.g. no heating in winter 50%, shower not working 20% etc. The tenant will only pay the full rent again from the moment the damage is fixed. Of course you don't simply deduct but report to the landlord what is not working and give the landlord some days to fix it before you are entitled to reductions by law.