TY.  Have done investigation and found following; Fixer (Free)
Base currency MUST be EUR
https://fixer.io/documentation - "Exchange rates delivered by the Fixer API are by default relative to EUR." Fixer (paid)
Base currency can be any supported.

  CurrencyLayer (Free)
Base currency MUST be USD
https://currencylayer.com/documentation#source_currency_switching CurrencyLayer (paid)
Base currency can be any supported.
  ExchangeLayer (new coded, not yet in Core)
Base currency can be any supported.