Gosh you seem to have had a bad time of it.

Your best bet, when you had overstated your opening debtors would have been to post a batch credit note to that customer as a T9 entry and nominal code reserves.

Do not post journals to the debtors control as Sage does not like it and your Trial balance figure will not agree to aged debtors printouts

Have you checked your opening balance on reserves agrees to the accountants ?

If it doesn't agree , hopefully its out by the amount you're looking for ? :)

Did you post the opening balances on the sales ledger to a suspense account?

Seems the more you try to fix it, the more you make a mess of it. You could clear out sage and start again with the opening balances. Go to file > Maintenance > rebuild data This will wipe everything you have done and give your a fresh start to enter in the opening balances. However as its goes back to May 2012, this is not an option, unless you wish to re do it all

Remember, when entering opening balances, all balances except the creditors and debtors can be entered via a journal, the difference in this journal can go to nominal code 9998. The opening balances for the debtors and creditors, ideally, would need to be entered as opening balances within the customer or supplier account area. This will then net off in code 9998 to nil, leave you will a detailed transactional opening balance for each customer and supplier account and make your TB balance.

Did you do the opening balance reversal to Debtors Control via journal or via the customers account?

I think you will need to check what the correct opening balance should be for all your nominals, then work out if each one in sage is correct. You may find that the difference is either in 9998 (although this should be noticeable as £50,000 is not a small amount) or, it could be that your reserves are incorrect and the £50,000 needs to be corrected in the Debtors Control and the Reserves nominal code.