So sorry to hear that you are having such trouble with scammers.  The kind of scenario you describe is particularly rife on Gumtree.  There are some good private landlords on there but they are overshadowed by those trying to con you.

When you are looking for properties it might be better to deal with reputable property agents even if it means paying a fee as you are far less likely to get scammed for your deposit.  There are a few useful sites like www.rightmove.co.uk, www.FindaProperty.come , www.hotproperty.co.uk, www.foxtons.co.uk, www.primelocation.com etc.  It is easier to search on those kind of sites as you can specify your budget, property type and location.  That is how I found a great place. 

I've had personal experience of this.

I was looking on Gumtree earlier in the year and contacted a few people with places advertised on there. About an hour after I'd sent the initial enquiries I recieved a response from 2 seemilngly non associated people. One of the people said he was a missionary currently working in Africa, and I would have to pay a deposit before I'd viewed the flat. The second said he was a missionary somewhere else - but I'd also have to pay deposit before I could view. Deposit payable through Western Union.

I checked the IP of both senders and they were the same. I'm not naive enough to fall for that crap anyway but people who don't know how our letting system work may well have been. These people make out they're doing 'Gods Work helping poor people' to encourage you to trust them.
I replied to both emails, cc-ing the other into each email and had a good old rant. These people make me sick, scamming some unsuspecting person out of hard earned cash.
Be very careful,
Love Alex x

Yeah, I've noticed a few of those while I've been looking.

The one that worries me more is less common, but people sometimes rent a flat and then pretend to be the landlord, offering it to prospective tenants at a price that reels lots of people in and the running off with everyone's deposits.