Mettle is a subsidiary of NatWest bank that focuses on providing digital banking needs for sole traders and small businesses. They currently have over 130k customers, but KashFlow hasn't signed up with Mettle as a TPP (Third Party Provider) so Mettle customers can only use manual CSV export/import to reconcile accounts.
Mettle's Open Banking implementation is the same Open Banking API standards that other banks use so this should be a straight forward implementation.