In reply to Tricadam:
Good luck with your search, but I think that you are a little late in the season. Much of the best and most suitable (for you) instruction/courses for late March and April will already be booked up.
Alison Culshaw, whose course you provided a link to, is terrific. For you, her 4 x half day off piste courses in Chamonix would be perfect. Done it myself. However there is no availability for that, and much of her availability is fully booked by late summer.
So if you want her instruction this season you have to stump up for the full week including half the time on activities you seem confident that you have already mastered.
In Chamonix the other very high quality instruction (known to me) is Mark Gear of All Mountain Performance. He is fantastic. At your standard (as you describe) you would be at his Intermediate level, ie beginner off piste. His courses are full, and there are none in April. The advanced level would be beyond how you describe your skiing.
Your other option might be to check out BASS Chamonix. They provide good instruction, but I am not sure of availability
You always have the option of ESF. Out of school holidays they can be cheap, but a complete lottery.