This FAQ answer seems slightly odd. I don't know who would consider £150 or less to be a "lump sum" deposit, so presumably they're just using it for ad-hoc payments regardless of size.
https://www.natwest.com/savings/digital ... saver.htmlCan I make a lump sum deposit?
Yes, but the combination of lump sum deposit and standing order cannot exceed £150 per calendar month. After account opening you can make a deposit via Online or Mobile Banking.