Retention money returned to former owner before bill aettler

Hi all,
First time posting and may be a tad long... But really would be grateful of some advice.

When myself and my partner bought our leasehold flat in 2013, there was a bill outstanding for some major works undertaken to to the building before we completed. It was agreed by both solicitors that the money to cover this would be held in retention until the final invoice for the works was raised.

We received the bill in September of 2015, and I forwarded it on to our conveyance solicitors. We had a few email exchanges with some daft questions from the former owners solicitors, but we're told that the former owner and their solicitor were talking to the freehold company and we would be updated if there were further developments.

We heard nothing more, and the freehold company never chased for the money, and it didn't appear on any service charge statements received. So we assumed that the matter had been dealt with.

But earlier this year we tried to sell our flat, and found that the amount was still outstanding. This was detailed in the management pack we had to purchase from the free holding company.

We used the same solicitors when trying to sell, so we reminded them of the situation. After much back and forth we discovered that the solicitor had actually paid their client back the full amount,last sometime in 2015, and had since stopped trading. They have been bought by another firm, and the solicitor in question currently works there.

So basically we are left with this bill. We have complained to the SRA, and this week we received an offer from the solicitors without prejudice for half the amount. But we are not willing to take this, but worried if we don't we could end up with hefty legal bills as well as still having to pay the hefty bill to the free holders.

Any advice would be gratefully received. We have all email correspondence relating to this, and have the email from the solicitors agreeing to hold the retention until the final invoice was raised.

Are we right in thinking we should litigate here?

Thanks!