I though evershade's initial response was reasonable and didn't deserve to be dismissed as unwanted advice.
I doubt this can be debt considered as time barred if you failed to provide a forwarding address.
I doubt the solicitor was under an obligation to pass it on- anyway, by the time the demand came through did the solicitor have the latest forwarding address for you?
Have you contacted the solicitor to ask what he did with the request? I'm not clear what you want to get out of any complaint against the solicitor- are you asking whether he should now be liable for paying your arrears.
In summary, I'm not clear what you are asking.
I'm still not sure what advice you are looking for, as you say you do intend to pay the arrears. If you do pay them that's the quickest way of getting the debt collectors to stop threatening you. Or have I misunderstood- are you looking to avoid payment?