A cross-party group of MPs want an inquiry into the conflict-of-interest issues relating to the law firm Herbert Smith Freehills and its role as both litigator and later compensation scheme adviser in the Post Office's Horizon debacle.

The Post Office agreed in May to compensate 550 workers following almost 20 years of campaigning on behalf of postmasters that were wrongly accused and convicted of theft after using the group's faulty Horizon computer system.