Edit - the text you want to quote at them is regulation 34 para 5 of the Consumer Contracts (Information, Cancellation and Additional Charges) Regulations 2013:
(5) If the contract is a sales contract and the trader has not offered to collect the goods, the time [for a refund] is the end of 14 days after
(a)the day on which the trader receives the goods back, or
(b)if earlier, the day on which the consumer supplies evidence of having sent the goods back.
thank you so much for the advice, i couldn't find this on the which website, but really useful
turns out the post office had so much stuff for the shop that they have ended up making an additional delivery to them with multiple uncollected parcels and mine has now been signed for.
This Which page suggests that if you've got proof of postage back, they need to refund you within 14 days of the date of posting; presumably their inability to get to a sorting office is their problem, not yours?
https://www.which.co.uk/consumer-rights/regulation/consumer-contracts-regulations#cancelling-goods-and-services
Edit - the text you want to quote at them is regulation 34 para 5 of the Consumer Contracts (Information, Cancellation and Additional Charges) Regulations 2013:
Trading standards?