underpaid tax

333rocky333

Hi
Recently had my tax reviewed , it was wrong and they OWED me money.
I was wrongly paying tax on the company van.

However they noticed an under payment from 2004 -2005
I am employed and stopped tax ,and also have investments at 40% interest , they send me tax returns and i do them and they do the calcs.
I just dont understand how i can owe them after this amount of time.
Is it normal to be underpaid every year, is it the fact they dont know the amount of interest i will get that causes it.
I thought they just take whats owed over the following year.
Should I just accept it and move on.
I have wrote to them but had no responce.

I think I see where the £729.17 comes from but I thought the P2t forms meant they had allowed for this.
Thanks

If any help
I have the following
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SA302 03-04 PLUS UNDERPAID TAX £615.40
TAX AND INS DUE £233.50
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P2T 04-05 TAX UNDERPAID £495.67
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Received june 2007
P800T 04-05 INCLUDES UNDERPAYMENT £729.17
TAX UNDERPAID £647.44
Footnote says underpayment to be carried forward to 05-06
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P2T 05-06 TAX UNDERPAID £233.50
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Received june 2007
P800T 05-06 INCLUDES UNDERPAYMENT £647.44
TAX UNDERPAID £405.31
Footnote says investment income based on 04-05 year
underpayment to be collected during 08-09
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333rocky333

Received letter
underpayment included in tax codes for the 3 years ended april 06 were the self assesment balancing payments for 01-02,,,,02-03,,,,03-04 collected through the tax codes