this is a British Part I registration number (the same as is used for big ships). Probably when the boat was or newer (maybe 1960-1975).
If the numbers are sequential then, as said, the OP's, 703715, is from some time around 1990. (That's going by our similar number.) 1975 was, I think, around the 400,000 mark. Anyone any idea what it's up to now?
Latest Part I cert I have a copy of (as we sold the boat a year ago) is a 2010 boat with 700XXX