False promises & wasted time

Starting with the good, I already have one buy to let mortgage with Natwest. The application took about 4-5 weeks. Offer was granted and no issues. The transaction completed all fine.

However, 3 months later I decided to submit another buy to let application for a different property. This time things were bad on all levels. It took them 10 days to key in the application and only did it after I called twice. After 6 agonising weeks, they rejected the application based on affordability because they changed their stress test inbetween applying and decision. They also undervalued the potential rent by £50 a month. They told me to re-apply and add £10,000 to the deposit! Now, I have a lot of experience with other lenders. Most others would just amend the application. Anyway, I re-applied and added £10,000 to the deposit. 3 weeks later they tell me that it has again failed on affordability but this time no explanation given. I was told to seek independent advice. A total of nine weeks wasted. I have been their customer for nearly 20 years. I feel like running away.