Thank you guys, I am learning this stuff as I go along. The MiniGPS Reference Clock I use doesn't have an OCXO, only a TCXO. I have, however, another, rather underused unit with an OCXO, the Marconi 2022C signal generator, which, as I've discovered now, was about 2ppm off (after at least 20 years since the last adjustment/check, possibly more as I've bought it broken and fixed it about that time). I've adjusted it and it appears to be more stable than the OCXO in the counter (at least for now). If I use the GPSDO as a reference for the counter, the drift of the Marconi is below 1ppb in an hour, around 5ppb overnight with a few degrees change in room temperature.
Cheers
Alex