I’m really sorry to hear about the delay 
For clarity here, we are not a member of the Cheque and Credit Clearing Company so we don’t process cheques ourselves and our processing time is not included in the 4 day period. Sadly it can take a while for cheques to reach us, particularly if they get posted over a weekend or bank holiday.
Cheques are usually processed on the day we receive them and are bulk scanned through a fancy machine and noted as received on your account. They then go off to NatWest that day who start the process of clearing them. We then download a file daily from them and mark cheques that have cleared/bounced as such on your account.
Occasionally, we get big batches of cheques that we may process across multiple days but that is a rare occurrence.
In the future, we’d love to do in app cheque imaging but that is a fairly involved process and would require us to become a member of the Cheque and Credit Clearing Company I believe, so I don’t know the timescale on that.