Thanks for all the advice so far, although I'm less clear on what rights I have since yesterday. Would be good to get some clarification before I call them tomorrow, although I might get CA on the phone.
It's a bit of a mess finding out online isn't it? As above, it's correct that s20 deals only with goods not conforming. My bad. Although the Distance Selling Regulations were superceded it seems not by the CRA directly, but rather The Consumer Contracts (Information, Cancellation and Additional Charges) Regulations 2013. You still have 14 days, you still can't be charged a restocking fee, but you could be charged return postage if it's in the original contract (which you should have received a written or email copy of after completing the distance sale). If in doubt CAB would be your best bet.