I've had similar issues a few times over the years. This is how it panned out for me........
Your tax code is ultimately down to you as far as the Inland Revenue is concerned. I dont mean that you can make your own up:) , but it is up to you to check your coding is correct as far as they are concerned. An accountant is no more than an advisor, working on tax code information they are given by the Inland Revenue, the Inland Revenue provide this coding information to them based on the information YOU need to provide. If your company accountants provide the revenue with that information on your behalf (ie, you are not under self-assessment and your companies accountants do all this for you), you still need to check that information is correct. If that makes sense, you will appreciate that whilst this mistake can be blamed on your companies accountants, the Inland Revenue will simply say that you should have checked that your coding is correct and advised them if it was'nt. Not that easy for most of us, but thats the bottom line.
In these situations, (ie a mistake by your accountants, and not a blatant attempt to avoid payment of tax) the Inland Revenue will usually accept back-payment spread over a year or two if you ask them. They will provide you with a new special tax code that effectively takes a bit more out of your wages every month until you are evens.