I got my mechanic to check the spark plugs, ended up replacing them as per my request.
He insisted that BMW dealers definitely would have changed the plugs if they have charged for it but when we took them off today they looked like they came from another car. My car has only done 13000 km but due to the age I was requested by the dealers to keep log book service up to date ( they ripped me off ).
Anyway... Rpm needle doesn't drop intermittently and I'm monitoring fuel consumption , so far it's settled to where its meant to be. Regarding dodgy plugs, I have no way of proving what dealers did with the plugs. Later
I will post a pic of the plug which they have meant to be replaced a month ago.