What is meant by "habitual" prostitution. ?????
It appears to be a very grey area to me -------- it would mean that every independant prossie is breaking the law, that can't be right surely. ??
No Landlord will want to go into any "grey area" so as soon as he's alerted she's out of there
After all once it has been brought to a landlords attention he will not wish to run the risk of knowingly allowing it, to say a landlord will turn a blind eye is rubbish, possibly a foreign landlord letting out dodgy properties for cash, but no respecatable landlord would run the risk as it could also cause them issues with any Mortgage / borrowings they may have on the property.
As for putting a note through her door, why risk it, she may cause you all sorts of grief and hassle and still may not leave, reporting to the landlord causes no hassle to you and a guaranteed result.
If the Landlord doesn't act, then ask him...
Can you confirm that the insurance for the building allows for the apartments to be used for commercial purposes (i.e. prostitution)
Or contact the council and ask them if the Apartments are assesed for commercial use for council tax.