The best sugar substitute is aspartame (sold under the trade name of Sugar Free) and not saccharin. It is permissible with the medications you are taking and will do no harm.
Due to thousands of manufacturers (over 17,000) producing over 40,000 brands of various medicines one should select brands with due care from reputed manufacturers. Every now and then one finds newspaper reports of sub-standard drugs being sold in the market.
The following brands of atenolol appear on the international drug database: Aten (Cadila), Tenolol (Ipca) and Tenormin (Nicholas).
Since you are concurrently taking isosorbide-5-mononitrate (Monit-SR) and aspirin (Ecosprin), it is preferable to take Solosprin (Zydus Cadila) that contains both.