My mother always says (sarcastically / indulgently)
"I'll believe you, but thousands wouldn't".
(We're australian, UK ancestry, my mum is in her late 60s)
EDIT: Aha! It's not just my mum:
The following is from "The New Partridge Dictionary of Slang and Unconventional English" found on google books
I believe you but thousands wouldn't
- a catchphrase retort that is used to express doubt or, at best, reserve judgement about the veracity of the person being addressed.
- Quotation: "It's the God's honest truth I'm telling you, Johnny". Mellors stood frowning down at him for a moment, then he said,"All
right kid, I'll believe you but thousands wouldn't. Now sleep it off"
(Derek Bickerton, Payroll 1959)
The following is from https://en.wiktionary.org/wiki/I_believe_you,_thousands_wouldn%27t
I believe you, thousands wouldn't
- (Britain, Ireland, sarcastic) Used to indicate that the speaker does not put faith in something they have just heard.
- Quotation: 2004, Sandra Newman, The Only Good Thing Anyone Has Ever Done: A Novel, →ISBN, page 185: You weren't working on germ warfare
five years ago." "Non-lethal," she cries. "Oh, how you can not see."
"Well, I believe you, thousands wouldn't […]
I've always liked this phrase because it's kind of sweet how it doesn't accuse the person of speaking rubbish outright :)
In fact, my mum said this to a number of tall tales I relayed to her from classic works of literature such as "1001 crazy facts from around the world for kids", which although I now realise were more 'sensationalised-and-hard-to-disprove-historical/urban-myths', at the time, I was quite wounded by her implicit accusation, and would launch into a passionate defence of my source, yet I also felt a bit smug because, like the phase taken literally, I knew something which was quite obscure, and likely thousands actually wouldn't! :)