Hi Marc,
From what you have just said it certainly seems to be the clutch and if you have to dismantle it you will need a puller, you can get by without any other special tools but not the clutch. Just had a thought, under the 100mm cover held on by 3 screws there is an adjusting ring and if they are not tight maybe the ring has moved. I don’t know, just a thought. Anyway good luck, Alan