Very difficult. You can apply to Court to under Section 263(3) of the Insolvency Act 1986 [application re actions of Supervisor]. The Court can make such order as it thinks fit including, in an extreme case, replacing the Supervisor.
Trouble is, it's expensive, and replacement of the Supervisor will be the ultimate sanction.
Bear in mind that whilst you may deal with a large company, it is an individual who holds the insolvency license and it's he or she who will face any disciplinary action for example. Bear in mind that he or she may be blissfully unaware of what is happening. If you have the e-mail address of anybody working at the firm (say
fred.bloggs@x.co.uk) apply the same format to your supervisors name and you should be able to e-mail him or her.
Spell out your concerns and you are much more likely to get somewhere.
Ian