How did the wily British put the noose around the neck of Nawob of Bengal?

Aliwardi Khan,Nawob of Bengal),India. www. indianetzone.com

In 1756,  Siraj-ud-daulah, his grandson (daughter's son) became the ruler of Bengal. He was young, incompetent and lacked  qualities of leadership. Since the time of Aliwardi Khan, earier ruler,  the British had been purposely violating their trade privileges granted to them by Farrukhsiyar in 1717 – building further fortification, strengthening their garrison at Fort. William, Calcutta without written permission from the young Nawob. The East India company's senior officials behaved in a manner that was quite insulting and irritating. Consequently, their credibility was the casualty. The company made a huge profit by fraudulent use of Dastak (free trade pass) which saved them a bundle. They carried on Customs free trading activities uninterrupted. Export-import taxes remained unpaid for a pretty long time and they had no idea to clear the arrears to the Nawob.  For the British company, it was a great financial gain. On the other hand for the  Bengal ruler it was a  great financial loss.

Bengal Nawob durbar and the British. credit: www.colombia.edu

To cap it all, the British became too nosey and their over interference in the Nawob's administration and insulting and insinuating responses angered him very much. This, obviously led to provocations and counter provocations on both sides, taking  animosity and hatred at much higher levels between the ruler and the arrogant British. Having left with no other choice, at last, the Nawob was forced to take a decisive step to teach a  lesson to the British who were brimming with arrogance and pride.  On June 4th 1756,  the Nawob's army captured the company's factory at Kasim Bazaar and on 20th seized Calcutta. This resulted in 'Black Hole (of Calcutta) tragedy and the casualties of British soldiers became a controversial matter. This incident became a trump card for the British to justify their stay in India and their military actions against the Indian rulers. This paved the way for the entry of Robert Clive into Bengal from Madras who was fresh from victory over the French led by Dupleix along with Chanda Shaib.

In 1733, the Nawob of Bengal  complained  about the English traders and their untrustworthy British company. It was something like this:

...When they first came into the country, they petitioned the then government in a humble manner for liberty to purchase a spot of ground to build a factory house upon, which  no sooner granted than they built a strong fort, dug  a ditch  around it to have  communication with the nearby river and mounted a great number of guns upon the walls. They  enticed several merchants and others to go along with them and take protection under them and  collected a revenue which ran into Rs 10,00,000.00 ... ..... they robbed, plundered and carried great number of the king’s subjects of both sexes into slavery in their own country …...A total breach of trust and confidence on the part of East India Company.

Map of early Bengal credit www.  leics.gov.uk   

After the death of the Mogul Emperor Aurangzeb (on 20th February,1707), who openly discriminated against the Hindus and engaged in several wars against other kingdoms, especially Marathas ( which drove the Mogul Empire to the brink of bankruptcy), an ideal situation developed for the  British to take control over Bengal using various excuses. In fact, the British traders had been patiently waiting for this wonderful opportunity for decades. They never thought that such a situation would brew to the brim so soon. While rest of India was replete with turmoil- fratricidal wars, Maratha invasion, Jat uprising, etc., Bengal was quiet and well administrated. 

The recent devastation caused by the unexpected invasion, plundering and looting of vast treasures by Nadir Shaw from Persia left the Mogul ruler Mohamed Shah bewildered and exasperated. It took a beating on Mogul's power and prosperity. Cracks developed in their powerful hold on the Indian subcontinent. While elsewhere, particularly,  in many north Indian kingdom a chaotic situation was prevailing, Bengal under Alivardi Khan, had been prosperous, peaceful and paradise; well progressed in agriculture, industry and banking as was illustrated by Jagat Seth, banker's banker;  a haven for wheeling and dealing.

At last, the British  reached the final stage of putting the noose around the Nawob' neck  and his country after the battles at Plassey (June, 1757) and Buxar (October,1764). All they had to do was to tighten the rope around the neck. By being dishonest and cheat, the British company made a sucker out of the Nawob of Bengal whose  business dealings with them were honest and without any blemish.
Ref:
https://en.wikipedia.org/wiki/Bengal_Presidency