Having just had lunch an hour and a half late, being a diabetic, not a good idea, I can be more responsive.
Here is Accountancy Live's advice on the recovery of s.455 tax:
'Getting the s455 tax back
Section 455 is effectively a stand-alone tax charge, which is ‘deposited’ with HMRC. If the loan or overdrawn DLA is wholly or partly repaid (or released), the appropriate portion of the s455 tax is discharged (ie, effectively cancelled) or refunded. This section initially looks at the amount of the loan outstanding at the company’s year-end. However, provided the loan is repaid within nine months of the year-end, the s455 liability is effectively cancelled and the tax does not have to be paid over.
In practice, the company often avoids the payment of an actual s455 tax charge by arranging for owner manager to draw an appropriate bonus or dividend, which is credited to their loan account within the nine-month period. However, if the loan remains outstanding beyond the nine month due date, HMRC will seek the s455 tax and charge interest from the due date until the tax is paid. Section 455 levies a tax charge, equal to 25% of the advance or loan, where a close company makes a loan to a participator or their associate When the loan is subsequently repaid, the repayment of the s455 tax is deferred until nine months after the end of the CTAP in which the loan is repaid or reduced (s.458).'
If this loan is not going to be repaid as it would appear then the director's could resolve to write it off to the profit and loss account as an exceptional item which would have to be separately reported in the statutory account. This would remove the loan and open the door to recovering the s.455 tax paid.
Personally I would use a tracing agency to locate him or her and sue them for this substantial sum in the appropriate forum, the County or High Court (Sheriff or Court of Session in Scotland). However this would need the approval of the directors and they may not wish to so proceed. What the shareholders might say is quite another matter, but probably these and the directors are one and the same!
Whether or not the past director declares dividends on his own tax returns is a matter for him and HMRC; the company is not involved.
I do hope I have been able to assist you with your problem and shown you a way forward.