HMRC errors during a compliance check - advice please!

I would very much welcome advice from those experienced at dealing with HMRC.

The situation is as follows. A client's 08/09 return is currently the subject of a "compliance check" because the amounts entered on the return didn't match the employer's returns. Turns out that the client received payments after his P45 which, by the time the return was prepared, he'd forgotten about, so the P45 was used as the basis for the return. The payment after the P45 was only subject to BR so there is some higher rate tax missing.

We have replied to HMRC agreeing that the extra tax is due. Brief explanation along the lines that Client had been made redundant very suddenly, whole event rather traumatic, no supporting paperwork from ex-employer advising that the payment was taxable (rather than being tax-free redundancy) and since he was made redundant in May 08 there was a long time elapsed before the return prepared.

Letter received back from HMRC saying that they think this is a "failure to take reasonable care". We didn't really disagree UNTIL we reviewed their calculations to find that a completely random wrong number (no obvious typo type error) has been used as the top line "income from employment etc"  and that this has increased Client's supposed liability by nearly 30%.

My point in all of this is that if the client had not had an adviser there's little chance he would have doubted HMRC's figures, and would therefore have paid much too much. Surely he should be "entitled to rely" on the figures given by HMRC? Have they, too, been guilty of "failure to take reasonable care"?!

If so, is there any way I can use this to get my Client's penalty level reduced? Any experience along the same lines would be much appreciated.