Contractor Among Contractors
Advice can be got from any competent accountant or tax adviser. I would say however that if you got into the scheme via one of the above, don't go back to them as they are potentially conflicted.
This is however a specialist area and whilst the appeal and postponement application in itself is straightforward, dealing with the next level of enquiry is not.
To do that requires some expertise and access to knowledge and history.
There are rules here about promoting any particular firms so I suggest that you have a look around and see who is being used.