Was just about to echo what futuredad wrote, although you may have been served notice before, some landlords will do that to avoid the exact problem futuredad described.
If you've not been served notice, then you're in a much stronger position, you could just up and leave on the anniversary, you could keep paying the same old rent until you were served a valid notice of change (and you could still leave then of course, 2 months notice required almost certainly)
You may be getting a "good deal", but then again the landlord is getting a pretty good deal too, if he decides to try and increase the rent, he might make 875 quid over the year. If he decides to kick you out, he'll probably not get any new tenants until after you've left (you're not going to be very helpful about viewings, and will certainly tell everyone who visits what an arsehole the landlord is having you do free improvements then kicking you out), So he'll likely lose a months rent, and pay agents fees losing him a lot more than 875 quid.
In reality he's probably a perfectly nice guy, the agents are just trying to get a load of fees for a new contract, and make themselves look good to him by getting a rent increase. Just contact him directly, tell the agents to bog off and negotiate your own deal, either nothing or a bit more depending on what you feel like.
Remember if there are 1200 flats being advertised they're probably not going for that in southwest London my last three flats have all been accepted over 10% below what was advertised.