The time bar period would be five years if your contract with them was under Scots law and six years if under English law. If no attempt was made to take you to court and there was no admission of debt during the period then from your narrative the claim is time barred as well ignoring your claim for outstanding commission. Write to the solicitor denying the claim in the basis of time bar and counterclaim. I doubt you will get a coherent reply. Happy to discuss but I think your suspicion about time bar is correct. I hope that helps. Please leave a positive rating so I am credited for my time.
The time bar kicks in if there has been no court proceedings raised within five years and no admission or acknowledgement of the debt within that period. So no the solicitors letter is irrelevant. It's a question of no action for five years from the obligation to pay. I think the case may already be time barred.