Thank you Michael, WZZZ and Varjak paw (again).
Okay. There seems a concensus forming. The drive is a Lacie 500 GB used for Time Machine. The 150 GB is what had been used.
I have to admit I chose the 7 pass option because I do tend to be over cautious in these matters. It did ponder how much longer it would take and wasn't overly bothered when the first time estimate showed up as 16 hours. But as time passed and the estimate rose from 1 day to 2 days (currently 2 days 2 hours). I suppose best policy would be to leave it running, and report back in 2 days time if things look no better; as of now it has been running for 10 hours and I'm on pass 2 of 7.
Most grateful to everyone who is contributing; any further observations would be equally appreciated.