Why are the inland revenue allowed to back date interest on income tax they beleive was not declared?

[Goodwill]

The revenue made an enquiry on my self assessment year end 2004-2005 after consultation with my accountant they decided to broaden there enquiry to cover self assessment returns between 2001-2006, they claim i have committed an offence under S95 taxes management act 1970 and that i am negligent in dealing wih tax affairs during the dated years and as a result have attracted a penalties which consists of additional duties,interest and net penalty. My accountant has been involved through the investigation yet i feel misrepresented as i have produced accounts/receipts for each year and he has produced accounts which i thought were satisfactory based on his calculations.I would appreciate comments.
7 seconds ago - 3 days left to answer

[factor-fiction]

Are you looking for us to answer the question given in the title (Why are the inland revenue allowed to back date interest on income tax they believe was not declared? or just for general comments?
If you have underpaid tax you have had the benefit of that money , and therefore the potential to earn interest on it. HMRC has missed out on the chance to earn interest while yo held onto money owed to them. They can now charge you an amount for their lost interest.

[buildersmate]

So Factor has covered off that bit - which presumably you agree with since he got three stars.
Now the second question I'm sure you are asking yourself is whether you have a claim against your accountant - to which the answer may be yes - if you give him all the info at the time, and he made an assessment on your behalf which now he is presumably agreeing was wrong. If you gave him incomplete info, then that's different.
At the very least, your accountant should be reimbursing you for the interest and penalty charges.