Amazon charge £6 for return via Hermes/ Evri collect!

Highly unlikely to be across the board "new policy", more likely a misunderstanding of return presentation options or some other misinterpretation / misrepresentation of reality / ambiguity of accuracy that is difficult to convey from a chat jockey that may have less than ideal comms skills.

Don't know the precise circumstances of the fault triggering the return but in general:

the consumer has no obligation to return a faulty product - just make it available for collection
but if agreeing to return:
the trader must bear any reasonable costs of returning them
or similar ramble lifted from Consumer Rights Act, possibly S20, can't immediately recall.
Highly unlikely Amazon has introduced a new policy to actively flout that legislation.

The consumer can of course decline any zero-cost return option provided by the trader and use any other mechanism at the consumer's cost and risk.

Was it supplied and sold by Amazon or market place?

"Due to new corporate greed, Amazon will now be charging customers for mistakes made by the company"

Just checked online. I got offered to have RM pick up at home, or various drop off options and all were £0

Was it a large item or one of particularly high value?

Amazon have gone awful lately. A few issues with returns. With the last one I tried to cancel an order but I was told I couldn't even though it hadn't been dispatched. I returned it the day after delivery and was told 5-7 days for refund. After 7 days no refund and was told it can take up to 14 days. 14 days now up and being told to wait 30 days! At least I didnt have to pay for the return

It depends on the option you pick when creating for the return . If you click Incompatible or not useful for intended purpose all the options will be £0