Harry, I'd love a set of Sea Commander plans.
My Sea Urchin is so old there's not a chance the plans still exist for it. This year it is 59 years old! I restored it 2 years ago and it still has its original switch, motor, shaft, coupling and twisted tin prop. Free running, of course. My Crash Tender is 55 years old and I still haven't quite finished it, although a nice birthday present of a set of SLEC fittings for it will make that happen soon. That has a Taycol Supermarine and original shaft and coupling. The REP r/C was stolen, as was the original Mersey Marine fittings kit.
So, sorry, can't help you there, but Ivan may have a set of Sea Urchin plans from before he flogged it all off to SLEC. I was in the process of doing further kit designs for him when he sold up suddenly. Haven't seen him since.
Cheers,
Martin