Please read my answer, about consumer rights, here:
http://www.theanswerbank.co.uk/Shopping/Questi on431861.html
That question is about a camcorder but exactly the same will apply to tents. i.e. if you can show that the tent is defective
because of an inherent fault (e.g. because the manufacturer failed to adequately waterproof it), the retailer is obliged to remedy the defect by offering a repair or replacement. (You have no right to a refund because, after 11 months, a court would almost certainly rule that you'd legally 'accepted' the tent, despite the fact that you'd not used it during that period). The retailer would also be responsible for meeting your travel or postage costs.
However, what I've written assumes that the tent was meant to be waterproof in the conditions under which you used it. If, for example, it was described as having an 800mm hydrostatic head, you could only expect it to remain waterproof in light showers. The tent could not be regarded as defective if it leaked under more severe weather conditions. (A tent with, say, a 1200mm hydrostatic head could be expected to remain waterproof under most normal UK weather conditions but you'd still have no cause for complaint if it leaked when subjected to a lengthy torrential downpour. If the hydrostatic head was quoted as 2000mm you could reasonably expect it to remain waterproof in the worst UK weather conditions).
Chris