Ok - reporting back from my field studies. I received my free sample of the Instead Soft Cup and here's my report:
Insertion - not too tricky, since I've been using the keeper for a few years and it was very comfortable. I also watched a video on the Instead website which was very helpful.
Performance - erm. I only tried it at the start of my period and I didn't get any leaks, but ... when I went to the toilet it literally poured out as I sat there!! Not sure this is meant to happen, especially if I hadn't been sitting on the toilet, so it put me off a little. I only had 2 in the free sample pack so didn't really get a chance to experiment. Might splash out on a whole pack and give it another go.
Summary - mixed. Needs further trials for a conclusive result.