>>> A strongly worded email followed up by a letter quoting distance selling regulations may do the trick
Well I certainly wouldn't quote the Distance Selling Regulations, as they've been repealed by the the Consumer Contracts (Information, Cancellation and Additional Charges) Regulations 2013!
However the relevant law to quote in your letter is actually
Section 29 the Consumer Rights Act 2015 anyway:
"A sales contract is to be treated as including the following provisions as terms . . .
The goods remain at the trader’s risk until they come into the physical possession of—
(a) the consumer, or
(b) a person identified by the consumer to take possession of the goods . . . "
http://www.legislation.gov.uk/ukpga/2015/15/section/29/enacted