From the clue in cell 8, we know that cell 18 is c.
We also know that the corner cells contain E, I, O and U. The letter E must be in the fifth column, since there is only one step between C and E (C-D-E and sporC-L-E.) Therefore, either cell 21 or cell 25 is E, and the two cells in between C and E are D and L (cells 17 and 22 or cells 19 and 24.)
If F has to be adjacent to E, it can only be in cell 16 or 20. Now, we know that the other corner of the fifth row is O (spO-R-Cle), since there is only one step between O and C. If we take a look at the letter I: there are two steps between the F and the I (F-G-H-I). F is either in cell 16 or 20. If F is in the first row (cell 16), I must be in the first row as well (cell 1). The same goes for the fifth row, if F is in cell 20, I must be in cell 5, otherwise there would be no possibility to position G and H.
This indicates that E and I are in the same row, and therefore O and U are in the same row as well. Now, we know that T is not in row 1 or 2, so U cannot be in row 1, since T and U are adjacent. Therefore, cell 5 is U, cell 25 is O, cell 1 is I and cell 21 is E.
Logically, cell 16 is F. Now we know that cells 17 and 22 are D and L, we can try to position those letters. There are 2 cells between I and L (I-J-K-L), so L cannot be in the fifth column. Cell 17 is L and cell 22 is D. We also know that G must be in cell 11 or 12 (adjacent to F), H in cell 6 or 7 (adjacent to G and I), J in cell 6 or 7 (adjacent to I and K) and K in cell 11, 12, or 13 (adjacent to J and L). L is in the second row, and O in the fifth. Therefore, M must be in the third row and N in the fourth row. N must be in cell 19 or 24, just like R (adjacent to C and O). If M would be in cell 13, then cell 23 would be completely isolated, and no letter would be able to be put there. Therefore, cell 23 is M.
P is adjacent to O, and not in cell 19 or 24, so cell 20 is P. If cell 19 would be N, and cell 24 R, R would not be adjacent to Q and S, which is impossible. Therefore, cell 19 is R and cell 24 is N.
Let’s take a look at cell 2. Cells 6 and 7 are H and J, so cell 2 has only one neighbor which has to be determined. Therefore, cell 2 must be Y, and cell 3 must be X.
With the clue from cell 3, we know that K and W are adjacent. W can only be in cell 4 or 9 (adjacent to X), and K in cell 11, 12, or 13. There is only one possibility: cell 9 is W and cell 13 is K. The only possibilities for cells 11 and 12 are B and G. B must be adjacent to A and C, so cell 12 is B and cell 11 is G. J must be adjacent to K, so cell 7 is J and cell 6 is H. K is not adjacent to Q, but Q is adjacent to R. Therefore, cell 15 must be Q. The letter S must be adjacent to P and R, so cell 14 is S. T is adjacent to S, so cell 10 is T. Only V remains, so cell 4 is V.