His legal team allege that the Post Office’s decision to pursue a civil claim against him was an “abuse of process of the court.” And that the eventual judgment against him was obtained by fraud.
They also all claim the state-run institution conspired with Fujitsu to pervert the course of justice by “deliberately and dishonestly” withholding evidence.
This included knowledge of bugs and errors as well as the issue of remote access – the ability of some Fujitsu employees to access sub-postmasters’ branch accounts without their knowledge.
The Japanese owned company developed the software and is responsible for operating and maintaining the Horizon IT system.
Mr Castleton was one of the 555 sub-postmasters who took part in the landmark court case against the Post Office and won.
Both sides agreed to end the legal dispute. But Mr Castleton claims the settlement doesn’t apply to his current claims as well as alleging it was obtained by fraud.
Specifically, he argues the Post Office concealed the true reason why the former Fujitsu software engineer, Gareth Jenkins, wasn’t called as a witness at the trial.
Mr Jenkins provided testimony in a number of prosecutions. But in 2013, the Post Office was warned that he had failed to disclose information “in plain breach of his duty as an expert witness”.
The sub-postmasters weren’t told about the concerns as they fought their case.
Mr Castleton is seeking both the civil judgement and the bankruptcy order against him to be set aside on these grounds.
A Post Office spokesperson said: “We recognise the devastating impact of the Horizon IT Scandal on former postmasters like Mr Castleton. Post Office today is committed to doing all we can to help those affected get closure.
“We cannot comment on ongoing legal proceedings but are engaging fully in the process.”
Fujitsu declined to comment to the BBC.