yeah this kind of problem does happen in many property sources. At the moment the users can report "Expired ad" so those properties won't appear again. We are brainstorming hard to tackle this tricky problem.
Couple of ideas you're welcome to (I played with exactly the same idea you have a few years ago but couldn't be bothered in the end):
The distinction between private and commercial ads isn't always clear, particularly on Gumtree where a lot of the time when you call a supposed private ad, it's actually an agent. You could add a "blacklist" of mobile telephone numbers which are known to be agents so if you view a private ad link there should be a link to "report this as a commercial property agent" and then after a threshold, mark any scraped ads with that number as commercial. Some people prefer going directly as the agency fees and service is utter crap and eats into your property budget pretty quick.
Add your own private adverts as well so you're the best augmented source of properties there are. Be the agent :-)
Also, don't just target the word "flat" - it can be a seen as a negative for non-flats so you're excluding some of your potential already. There is a lucrative market in high end housing which you could charge click-throughs on later to support yourself!
"You must not copy, transmit, modify, republish, store (in whole or in part), frame, pass-off or link to any material or information on or downloaded from this Site without our prior written consent."
Have you taken a licence from Rightmove et al for the data usage?
Heh, nope, but it's an insubstantial part of the whole and thus wouldn't constitute infringement.
Plus my pasting of a company's terms to ask whether the OP is in compliance is hardly likely to attract the ire of the company, whereas if I was Rightmove's lawyers I might have an issue with someone taking parts of my database wholesale for usage on a competing site.
I personally think a competitor aggregating results is a positive thing, but I was just curious as to whether this was done with a licence or not.