As you've changed the air filter and plugs etc, I would start with the basics.
Check that all the throttle cables are seated correctly at the throttle bodies.
Look for air leaks in the inlets (before and after the throttle bodies) - if you spray WD40 or brake cleaner around the throttle bodies with the engine ticking over, the revs
may rise if there's a leak (but not every time - do visual checks as well) - however be careful on the left hand side - you can mistakenly spray it into the air intake just under the tank which will have the same effect.
Are the little rubber bungs that seal the throttle body vacuum connections in place?
Is your exhaust standard? Any other 'tuning' modifications?
Is your bike a twin spark? (I've no experience with twin spark BMW's but a failing stick coil can cause running problems - not sure if the symptoms will be as you describe though).
After this, the slightly more technical things (which you may or may not be comfortable doing yourself).
When were the valve clearances and rocker end float last checked?
It might be worth checking the TPS voltage.
Is the CCP plug in place and the correct one for your bike?
The most important thing with anything like this is that if you are going to try anything or make any adjustment, be methodical and try one thing at a time so that you can be sure exactly what you've changed (and the effect it's had) and can undo the change if necessary. If you start messing with random stuff all at the same time, you can end up in a worse mess with no idea how to get back to where you were.
Given that you're in London, it may be worth paying for an hour or so of Neil's time at
http://www.gsshop.biz/ - he's a BMW independent specialist and has an excellent reputation (well, for fixing bikes anyway
...) - he'll be able to set your bike up perfectly and could save you hours of scratching your head and trying things.
And your English is fine - a bloody sight better than I'd do trying to discus anything technical in a foreign language
You will get some sense in a moment I'm sure. 
