I have just open a Nepalese and Indian Restaurant in London and it used to run by different name and different management. They used to have nearly 200 customers so I want them to know that it's now under new management and new concept.
How do I write them letter please?

Regards

These days letters of introduction are pretty obsolete.
People toss a lot of mail without even opening it.
Have postcards made with a picture of you and your new sign and restaurant, and say 'New Owner, please try our delicious cuisine and get 15% off (alcohol excluded) now until May 1 as our thanks to you' or somesuch.

Thank you mate! I will do that