emyr,
Bleach interferes with luminol, but the effect dissipates in less than a day, IIRC. There are some papers about it in the forensic literature (link to one
here). However, it is the only substance of which I am aware for which this is true. I cannot see why one would need to wait six weeks. Here is a quote from the paper by Kent and colleagues (J Forensic Sci. 2003 Jan;48(1):64-7.): "If time allows, an alternative method for reducing interference from hypochlorite bleach is to wait several days until the bloodstains have dried thoroughly, by which time the hypochlorite will have decomposed."
ETA
The paper by Creamer and colleagues entitled, "Attempted cleaning of bloodstains and its effect on the forensic luminol test" in the journal
Luminescence, Volume 20, Number 6, 2005, is a good resource.