We refer to your letter no. _____________ dated _____________ regarding the request for CD and Publications / materials on the model and operations of IR in Delhi.
In this regard, we would like to inform you that this project has been executed by the NIR, and, therefore, these details can not be provided by us. You may kindly write to the Managing Director, NIR, ________________ (complete address) to seek these details.
It is not normal practice to number a letter, therefore Ishould be omitted.. We refer to your letter dated.. regarding ......is sufficient.
executed = I would use undertaken
We suggest that you write to the Managing Director, NIR ... requesting the details you require.