AFAIK both forms are acceptable [in en_GB] for both the past participles of caulking your boat and making a financial transaction, cf http://en.wiktionary.org/wiki/pay#English or http://oxforddictionaries.com/definition/english/pay--2.

This IMO highlights why one shouldn't get too anal about specific word forms as long as the intent is apparent. Of course one can generate inaccuracies this way but generally an honest attempt to receive the communicated information doesn't gan aft agley.