Both sides struggled on the wet surface but the visitors claimed the points when Will McCall stabbed home the loose ball from close range with just eleven mimutes remaining.
Blues started well and had shouts for a penalty turned down when Jimmy McCarthy went down in the box.
Ilkeston forced a save out of Danny Roberts just before the break when he scrambled a skidding low shot around the post.
McCarthy had an effort blocked early in the second half and Devon Gibson broke away down the right but could only shoot straight at Gathercole in the Ilkeston goal.
Ilkeston gradually got on top as the game wore on and after McCall had broken the deadlock, substitute Tyrell Waite could have wrapped things up but he blazed over when through on goal.
In truth, Blues didn't seriously threaten an equaliser and Ilkeston held out for the victory that took them back to the top of table.
One consolation for Leek was the crowd of 536, which was the largest at Harrison Park in over four years since 550 turned up to watch the league derby with Buxton in September 2007.
1. Danny Roberts
2. Tim Sanders
3. Ricky Bridge
4. Darren Tinson (15.Ben Nixon, 84)
5. Glyn Mellor
6. Michael Thomas
7. Jimmy McCarthy
8. Sean Turner
9. Matt Haddrell
10. Christian Millar
11. Devon Gibson (12. Tim Grice, 84)
Unused subs: 14. Ashley Woolliscroft, 16. Oliver Edwards, 17. Wayne Corden
Cards: Jimmy McCarthy, Sean Turner
Man of the Match: Ricky Bridge
Photos: Hampton Productions