My wife and I jointly own a small number of buy-to-lets, and ten years ago my accountant completed a Form 17 declaration to HMRC to have the income allocated to my wife.  Around six years ago, we bought three more buy-to-lets but did not complete further Form 17s although income continued to be allocated to my wife.  The accountant mentioned this and I told him to advise me or just go and do it. New senior management at the Accountants  have advised me that I could be asked to pay tax at my higher tax rate. I am concerned that having employed a large accountancy firm and paid relatively high fees, I may now be expected to pay a large amount of back tax due to their errors.  Is there any way forward on this?

In a case called Rowland v HMRC, the taxpayer claimed they relied on their accountant, who made a mistake. The taxpayer won and this was accepted as a 'reasonable excuse' (see HMRC Enquiry Manual:

http://www.hmrc.gov.uk/manuals/emmanual/EM4110.htm. Page EM4110).  You can use this in your 'negotiations' with HMRC.  I don't know all the details of your case but seems to me, from what you have written, you have a good argument to support your case.