HMRC Appeal

I have received a tax bill from previously understated income from a Tax Avoidance Scheme I was on 8-10 years ago. I accept now that I owe some tax but wish to challenge the figure I owe. The Letter has statutory right of appeal. I have spoke to another company who have advise this:
The appeals process is 3 stages.
Appeal to HMRC - The initial appeal will be sent to the officer who has issued the closure notice
Internal HMRC review - Following the initial appeal we will be in a position to request an internal investigation into the decision reached
Tribunal Appeal - This is the first step in the appeals channel which is outside of HMRC, whereby an independent Tribunal may consider the appeal
Additional Step - Engaging in Mediation following the submission of a Tribunal appeal to agree upon a settlement of the liabilities.
The steps outlined above offer us an opportunity to challenge the figures
Also a review of my self-assessment tax account will be appropriate to ensure that there are no additional penalties accruing.

I have been quoted £5700 to do this appeal and so I am looking for a cheaper tax lawyer to do this for me, as I cannot afford that much. I need to put in the appeal within 30 days which is approx 20th Oct