You were (and remain) 'jointly and severally' liable for the debt. That means that both you and your granddaughter can be pursued for the debt, either together or individually. (It was entirely up to the finance company to choose who to take court action against. They obviously decided that they were more likely to get the money from you).
You can be chased for payment for the rest of your life. (The '6 year rule' won't apply unless the finance company simply stops pursuing the debt for the duration of that period. It isn't going to happen).
If you've failed to comply with a court order, telling you to pay up, the finance company can now go back to the court to seek enforcement of the court order. One way that they can try to do so is by getting a warrant of execution, allowing them to send bailiffs in to seize your property. That can work out extremely expensive for you. For example, let's assume that you've recently bought a £500 TV set. After the bailiffs seize it, it will be sold at auction but potential buyers probably won't know whether it's in good working order, so it might only fetch £80. After the auctioneer has taken his cut, and the bailiffs have taken their fee, there might only be £50 (or less) left. So losing your £500 TV will pay off only £50 of the debt. (i.e. if you owe £1000, the bailiffs might have to seize between £5000 and £10000 of your possessions to pay off the debt).
Alternatively, the finance company could seek a charging order, where they could reclaim the money from the proceeds of the sale of your house. Or they could apply for an attachment of earnings order, forcing your employer to pay a percentage of your earnings directly to them. Or they could seek to freeze your bank account by applying for a third party debt order. See here: