visa expired - but if you stay

a general question - if a tourist visa expires in UK and the person (from India) still does not leave UK - How does Immigration get to know about it ????????

They dont get to know about it. The UK Government do not keep any records on who comes and who leaves. There is no such database.

People just melt away into the black economy.

On occassions people are caught in raids etc and deported back but these are usually just staged events for the media and to allay public fears about "being swamped by foreigners".

that is strange ??? surely they get to know about it -
whos responsible for the saga - is it the sponsor ???????????

This is how illegal immigrants are formed. Most don't arrive by rickety boat in the middle of the night, tossed onshore by the swelling waves. They walk off an airliner and perfectly legally and appropriately enjoy their tourist holiday. Then they just happen to stick around.

But when they leave does the Govt make sure they don't come back again?

It's likely now, thanks to the "War on Terror" BS, at least in the US, you'll be blacklisted if you voluntarily depart after overstaying your visa (unless you have some legitimate reason for overstaying). Thus staying in the first place.

I know people here in New York--law abiding, hardworking, middle-class-aspiring people of Indian descent--who can't go back to the Caribbean to see their families because they are here illegally. Once they leave, they know they won't be able to return.

Sponsor is not responsible and nothing can be done against the sponsor legally if a visitor they sponsor fails to return. Its not even too hard to return back to the UK if you come over on an Indian Visa. Lots of people make lots of money in the UK, go back on their original passport and make fresh passports in India with different names/places of birth and when they want to come back to the UK simply get a visa. The staff at the Visa Section at the British High Commission just dont have the nouse to investigate anything to a decent standard hence lots of people who have overstayed are welcomed back to the UK whilst legitimate visitors are denied Visas.

No such things as blacklists. They are all media hype. I have heard of people who were refused entry into the US (too much drinking and causing nuiscance on flights) and after coming back to the UK they have got new passports and flown back to the US without problems (this is post 9/11, and so the US head of Interpol who was complaining about lack of passport checks in the UK should really start looking at what happens in the US).

Sorry to go off on a rant but it appears that from the fiasco we had earlier this year when visas were given willy nilly to Romanians etc the British Government are not too concerned about people coming to the UK with the wrong intentions.