Any question answered...

For a DSR return (that is, one for any reason but within the cooling off period), the default position is that the seller must collect or pay for return postage. However if agreed at time of sale (i.e. usually via T&Cs) they can oblige the buyer to pay - NOT at a later stage.

Just so. Paragraph 14(5) of the Consumer Protection (Distance Selling) Regulations 2000, as amended -

'Subject to paragraphs (6) and (7), the supplier may make a charge, not exceeding the direct costs of recovering any goods supplied under the contract, where a term of the contract provides that the consumer must return any goods supplied if he cancels the contract under regulation 10 but the consumer does not comply with this provision or returns the goods at the expense of the supplier.'

The contract can therefore provide for the goods to be returned by the buyer at their expense. It's also dealt with by paragraph 17 of the Regulations, and paragraph 17(8) in particular.