1. Legally, both you & your husband are liable for the full debt.
2. I assume that at some stage the Council went to Court and obtained a liability order. They would have had to do this before being able to get the tax deducted from your benefits.
3. If they had such an order, I don't think the 6 year statute of limitations applies, but it is clearly unreasonable for them to leave this such a long time before chasing you. (If the debt was a large one the amount which would have been paid by deduction from benefits is small and the payments would have spread over a long period, but the Council's records should show payments being received and they should have chased much earlier if they weren't getting any money. So far as this is concerned, their senior management should look into their procedures to find out what went wrong with them.)
4. You should attempt to get a proper check of records made by the Council - going to a more senior level of management if necessary - perhaps with support from one of your local Councillors (it would certainly be worth contacting one).
5. You can make a formal complaint to the Council - either in an effort to get the proper check done, or afterwards if you are not satisfied with the outcome of such a check, and then go to the Local Authority Ombudsman with a complaint of maladministration against the Council if you are not happy with the Council's decision.
6. While all this is going on the Council should take the alleged debt back from the bailiffs and not attempt to recover it until the problems are resolved. You will have to ask for this to be done.
7. The local CAB may be able to help you with all this.
8. It is much better if you deal with the Council in writing - keeping copies of all letters and making sure they are delivered - send by recorded delivery, or take by hand and get receipts.