I want to write the phrase "take a rain cheque" and am British.
Should I therefore use the British spelling of the word cheque, or respect the baseball origin of the phrase "rain check" and use the American spelling?
The entry in the OED is for rain check and it describes the spelling with cheque as rare. The derivation has nothing to do with cheques issued by banks. The check part is 'A token, usually a memorandum of receipt, a ticket, or piece of metal duly stamped or numbered, used for the purpose of identification, or as evidence of ownership or title'. If you write rain check, you have etymology and usage on your side.