My initial reaction was that this would not be possible and indeed this gem from Stewart Accouting Services Chartered Accountants confirms this and provides the following:
'Pre Startup Incorporation Expenses
According to Section 61 of the Corporation Tax Act 2009 (CTA 2009), assuming any pre-trading expenses are legitimate, “the expenses are treated as if they were incurred on the start date (and therefore a deduction is allowed for them).”
Expenses can be claimed for up to 7 years before a company starts up in business.
Therefore any expenses incurred wholly and exclusively in the course of setting up the business for which the company was later formed can be reclaimed.
Typical pre-formation expenses include: internet and domain name fees, computer equipment and software, accountancy and other professional fees, and travel costs.'
Regretably for you, what your accountant has told you is correct as you are out of time by some three years.
I am so sorry to have to rain on your parade. You would have had to put this expense through your self employment account which is now probably closed. You could put the belated claim before HMRC with the hope that that Dpartment might allow you to reopen a very old tax year, well out of time, but I suspect that they will not play ball. However, they might agree, in these unusual circumstances, to the sums being allowed against the company, but don't bet on it! I would suggest that you ask, after all HMRC can only say 'No,' in the worst case scenario. Of course, do this in writing, much too important to rely on verbal communications in such a complex issue.