Two more home defeats in four days did not help their cause, but manager Casswell (pictured) said: "The last thing we want is for the season to peter out because a lot of good things have happened.
"We have lost more home games than we have won, which does not augur well, but there are still a lot of points at stake."
Leek have 10 games left starting away to second-bottom Quorn tomorrow, and Casswell added: "The situation is still in our hands until it is mission impossible.
"We have two big games against Quorn and then Stamford next Tuesday and we have a good run-in."
Casswell said there was no "gloom or doom" in the dressing room after Tuesday's defeat by Grantham.
The manager said: "We were more than happy at 1-1 at half-time, but in the second half Grantham were stronger than us.
"But five minutes after the break there was a turning point when Adam Roberts had a shot cleared off the line when there was some debate whether it had crossed the line.
"But we kept going even at 3-1 when the game was effectively over."
Striker Matt Haddrell serves a one-match ban tomorrow, so Dan Cope could get a start up front alongside Tim Grice.