Good news for a change!
Last Saturday I received a grovelling letter (I won't bore you with all the details) from Marks and Spencer in response to my last letter to them. Included with the letter was a voucher for 80% of the purchase price of the jumper which shrunk. They have now refunded me 120% of the cost of the jumper which means that after taking into account the cost of posting the jumper to them and my bus fare into Chester to attempt to return the jumper to the store there, that I have about £4 compensation for my troubles. I am not complaining. At last, I have got my money back which was what I wanted all along, as this was the first jumper that has ever shrunk when I washed it. It has taken three months of patience and letter writing when the store should have refunded me in the first place. I have learned from this experience always to have an argument prepared in case things do not go to plan.
Thank you for your comments, suggestions and support. They helped to make me determined to see this through to its rightful conclusion.