Not sure whether Bunney's Bikes still offer parking; Freewheel on Hockley probably still do, but a potential problem with both is what happens if you arrive back after they've closed. A totally secure, 24 hour option is the carparks at Fletchergate or Trinity Square, with bike lockers at £1 per day or £3 per week. If you need to be near/at the station, the Bike Hub on Station Street, as mentioned by Jugglingphil, is probably OK.
Sorry I didn't respond earlier - I tried from work on Tuesday but got dropped out for some reason.