so you've not been reading up here in the last 3mts then?
you don't get sent to prison for a silly one off mistake
there are numerous threads here with the format of a letter that you can base a suitable response upon.
one of which is:
Don't touch them owe me £500 since January 2019 make excuse after excuse. Seem they always have software problems sending money out. Keep saying they will call back or email nothing been chasing it now for 6 mths the phone staff always have the same banter we will chase it up and get back to you then nothing!