tortoise (tough)
Difficult puzzles are hard to compose so the easy version has a frame stuck round it for those that like simpler puzzles.
HINT: You CAN do it. The 1 in column 24 needs a 1-X-1 in column 25. The only two possibilities are r9 and r16. Try with temporaries (or mental) and one soon fails.
I really didn't think it much tougher than the other one. The 7 in the first row was fairly quick to locate and most of the puzzle fell into place after that.
Thanks.