Tax geared penalties proportionate?

We have a client who is persistantly late in submitting returns, and where the tax involved is substantial, ie anywhere between 50k and 100k per annum. He has regularly incurred £100 penalties but we usually try to estimate the liabilities outstanding and he makes substantial payments on account before the end of Feb each year to avoid surcharges. HMRC have now levied penalties for failure to file by 31 Jan each year going back several years and are relating the amounts to the tax outstanding on return at 31 Jan. If in their wisdom the govt decided surcharges should be applied for late tax at 28 Feb, it seems somewhat harsh and dissproprtionate to levy a 30% "seriousness" penalty on the tax outstanding at 31 Jan, when this has been paid within 3 to 4 weeks and has not fallen foul of surcharge rules. We are talking about penalties approaching £30k! We are being invited to make an offer but I can see how the penalty regime is appropriate for discovery or following enquiry but £30k for tax being paid late by 3/4 weeks and where interest and flat penalties applied? I believe it is possible to argue Human Rights legislation as the penalty is seemingly dissproportionate to the "offence". We have succesfully used this argument on CIS penalties before but has anyway come across this situation
Andy Shady