Your contract is with the catalogue company.
The law requires that goods be as described, fit for the purpose and of satisfactory quality - Sale of Goods Act.
Your treadmill is not of satisfactory quality, so you should reject it and get a full refund. You should be refunded all carrriage costs and payments. As it has broken so quickly, do not accept repair and accept a replacement only if that is what you want.
Here is a template letter if you get no satisfaction by telephone:
http://www.consumerdirect.gov.uk/after_you_buy /making-complaint/template-letters/faulty-good s/