I happen to be in SFA and rent my own place out, so I'm aware of the rules in both cases. A tennant on a shorthold lease can by law make his own repairs to the rental property, if he can argue that the defect causes a health or safety issue, and that the landlord has not acted to rectify the problem within a reasonable time scale. The tenant can then write off the amount spent against the next rental payement(s). Your local citizens advice bureau will advise you on this.
Seems fair that if the DHE's maintenance contractor (MODern?) haven't responded quickly enough to a significant defect (which I believe is often the case) then a similar rule will apply to SFA. Difficulty will come witholding rental, since it comes straight from JPAC off your pay. I can imagine that being an adminsitrative challenge.
I've found the PFI properties very good for maintenace, although they are brand new houses with fewer problems.