I have been in a similar situation, but I had kept my skills up-to-date (always do that!) and I had contacts who recommended me, so I found it easier.
You do need to brush up your skills. First of all, focus. Don't look at Java and .NET -- your problem is already that you are a jack-of-all-trades -- pick one and mold yourself.
If you go .NET, pick a personal project and use the full MS stack (SQL Server, MVC for ASP.NET, Entity Framework, MSUnit, Moq, Unity). It's not necessarily the best set of frameworks for .NET, but it is the stack you'll find at a lot of .NET shops. Other places will be happy that you know an ORM, a unit-test framework, and an IoC framework.
While you're at it, brush up your interview skills. Learn by rote the basic, standard phone-interview questions for your chosen technology. You'll be surprised how far that gets you.
Now some bad news: I don't know what you've been earning but, if you're struggling to find work, you may well have been overvalued. And you've let your skills degenerate in the line you want to be in -- so even if you were worth that much as a manager, you may not be worth it as a programmer any more. You might have to be a bit flexible on wages, unless you can afford to wait while you brush up your skills again.
Onto some better news, how to find a job in London:
The best way is to have contacts. If you don't have them now, that's unfortunate, but you can still get them. Google around for user groups and meetups for your chosen technology in London. Also, get involved with non-technical development groups, like the eXtreme Tuesday Club.
Your very presence at one of these groups indicates that you're more dedicated to your craft than your average developer. Network well and you will find people who work at companies who are always looking for people.
Another fast but deeply time-consuming method is to make your CV public on a job site heavily accessed by agents -- jobserve is always my go-to site, but some of the ones you mentioned also apply. This will get you two days of phone-calls from agents trying to convince you to go for all sorts of jobs. But take some advice from someone who's been down this road.
- Buy a throwaway phone SIM so that, when you find a job, they can't keep contacting you.
- Be very sure what you want BEFORE you upload that CV. Tailor the CV accordingly.
- Don't even bother applying for jobs on those sites. Let the agents come to you.
- Be confident when explaining what you want to agents. Try to make yourself sound perfect for your perfect job, rather than suitable for any job.
- Make it clear to agents that if they try to push something other than what you're looking for, you won't work with them again.
- Stick to that, cause they'll do it anyway (I had one that sounded perfect, til I got to the bottom of the description and found it was in Bristol), but there are plenty of agents.
- Make it clear that they mustn't put you forward for a job without talking to you first. Being sent to the same job by multiple agents is a big no-no.
- That said, don't let them talk you into giving up information on other jobs you're going for, no matter how hard they insist. As soon as they know a company is looking for people, they'll start harassing them. Just ask "would you want me to tell other agents about jobs you're sending me for?"
- Know that, no matter how hard you play the agents, you will end up at some dead-end interviews. But it shouldn't take too long to find a job.
Finally, while the agents in London are still very powerful, the economy is such that a lot of companies are trying to bypass them and save their 15%. StackOverflow careers is a pretty good way of doing that, so put your CV up there and browse it for jobs.
This will be a slower process but not so overwhelming and will probably lead you to a better job.