Firstplus wont pay Ombudsman award

You have no right to offset but you have a right to set-off.
Moneyineptitude is right as they will destroy your credit rating before they claim against you in court (if ever), preventing you from giving your say in court as a defendant.

Since its an ombudsman final decision, look at http://www.fos.org.uk/publications/factsheets/enforcing-an-ombudsmans-decision.pdf and you will see the FOS have no role in enforcement.
I'd pop down to your local county court (with no sharp objects ) and ask to speak to the someone in the enforcement section about it. It essentially means sending in bailiffs.

I would suggest not using money claim online or starting new court proceedings. A Firstplus solicitor would be able to get the claim struck out over incorrect CPR (civil process) as enforcing a final decision is not starting new proceedings.
If it was an offer by the company after an adjudicators view that was outstanding, a county court claim would work as I've never seen a compensation offer by a bank being withdrawn (which would remove the liability).