Well there might be another solution - if Jack is in England or Wales.
Since the bed was represented to Jack that it would do wonders for his arthritis, this could amount to being a misrepresentation. If Jack has a claim under the Misrepresentation Act 1967, then he also has a claim under section 75 of the Consumer Credit Act 1974.
If after an attempt to mediate fails, Jack could raise a small claims action at his local county court against Crapmatic, or Glydesdale, or both - both are jointly and severally liable for the misrepresentation.
Jack's biggest problem is being able to prove the salesman's claim. That said, the action would be inexpensive to raise and the outcome would be decided on the balance of probability.
The OFT have published an excellent guide:
http://www.oft.gov.uk/shared_oft/reports/consu mer_credit/oft303.pdf
Small claims advice:
http://www.hmcourts-service.gov.uk/infoabout/c laims/index.htm