Yep, I've had a Musto version too for many years. Very well made and we find it comfortable for reasonable periods, however would probably get one with a firm (board) type seat if buying again, as there is less constriction of movement we think. The fixed ones don't feel as secure though!
Don't know if Musto do a fixed seat version but might look into it.
I have the Plastimo one, which was about £52, and it's fine - no "slipping about". Only recently I was asked which one it was, because someone else saw me using it, and thought that it seemed very good.
its worth considering a climbers harness. adjustable leg loops and waist belt mean you will not slip about in it as you are effectively strapped in. Climbing retailers often have a hanging point in store so you can try several and see which one is most comfortable when hanging. I have used a bosuns chair twice, don't know which make, and found them terrifyingly insecure after 25 years of climbing experience.
Ordered the Compass version,they messed up the order, then I found there was a problem with the D ring support... it slides. So now have the Baseline model, which seems substantial but haven't yet tried it.
With due respect Brendan, I use and recommend the Top Climber, as reviewed in Yachting World this month. Even a climbing klutz like me can get to the top of the mast. It is the yotting equivalent of a climbers harness.