To answer my own question, I have found this stuff:
"MID-CURE epoxy 15min (BSI203)" made by Bob Smith Industries (BSI)
http://www.align-trex.co.uk/bsi-epoxy-mid-cure-15min-128g-4-5oz-bsi203.html

…Which looks like a reasonable compromise. i.e. http://www.bsi-inc.co.uk/mid-cure-epoxy.html say allow "45 minutes before handling parts and 2 hours for full strength."
Amazon.com say it has a "15 minute working time"
Wait, the US parent website http://www.bsi-inc.co.uk/mid-cure-epoxy.html says
"MID-CURE can be worked for about 10 minutes, with adhesion beginning after about 15 minutes. Full cure in 2-3 hours."
In truth, a working time of only 10 minutes is still a BIT fast for me. However it is certainly much better than their "5 min" Quik-Cure™ Epoxy, which begins to set in 4 or 5 minutes, in my experience.
Zap sounds promising and will be top of my list if BSI is no good, but I needed the stuff TOMORROW and ran out of time for this decision.
I already use BSI's 5 minute epoxy which I find excellent. I like its strength, clarity, range of materials it bond well with and plus the way unset surplus can be cleaned up with Isopropyl alcohol, but too fast for this application.
I'll let you know how I get on with the BSI 15min Mid-Cure.
J
PS
FWIW, BSI also make a "30 minute Slow Cure", but I think that will be annoyingly slow "Full cure in 6-8 hours" but "cured epoxy reaches full strength within 24 hours". So realistically you can't touch it for a whole working day… :^(
Edited By John Smith 47 on 17/01/2022 13:46:21