I'd second @CharlieC comment there.
I got my sigma 125 back from Stuart on Friday, went ultra lightweight flywheel and clutch as a first upgrade. Been out for a couple of hours this weekend, and it does make a large difference to throttle response and urgency of the engine. Wouldn't hesitate to recommend, both from a product and a customer service POV.
In the end I decided to go with an AP clutch and TTV lightened flywheel. Also replaced the clutch cable, CRB and associated bolts and fittings too. I'll have my first drive in it tonight so will let you know how it goes. I didn't realise how tight/stiff my clutch was - I just got used to it, until I felt the new one installed. Revelation!
I'm at Anglesey on the 10th, so want to get some miles in to break the clutch in gently beforehand.
What a brilliant community this is, full of helpful and knowledgeable people. Kudos.