Who's liable for the refund here on a cancelled flight?

[Macneil]

We booked a holiday with Tui on their website and selected Easyjet flights as they were a better time.
Easy jet cancelled the flights during the recent manchester airport power cut debacle. We booked on a later Tui filght and all was well.We paid separately for that flight.

When I asked Tui about a refund, they said claim from easyjet or on travel insurance. Easyjet say they have refunded the booker 48 hours after the flight was cancelled - I think that's Tui.

I've had flights cancelled by Easyjet before and they are very striaghtforward, no goodwill, or apology, but a refund within 48 hours. They are adamant that they refunded "the Booker" on 25th, and have no obligation to provide proof or confirmation to me - I didn't book or pay, "The Booker" did. They won't even confirm it was TUI

Tui on the other hand appear utterly clueless. Customer service "advisors" who cannot string a sentence together and are unable to find records and the documents I've uploaded via their website. Unable to point me to the relevant t's and c's that they claim make it up to me to chase easyjet. Just generally low level call centre customer service that you seem to get everywhere these days

So I give a lot more credence to Easyjets position than TUI's but legally, who actually is responsible? Tui invoiced me, I paid them, but they absolutely refuse to engage, just referring me to easyjet, full stop.