Two of my iSUPs have a smallish hole in the side. I have repaired as per instructions - they hold air for a few days then the patch seems to bubble and the board slowly deflates - looking for recommendations for a better repair -they are both good quality boards.

"They are both good quality boards". apart from the holes right? How old are they? They all start failing over time.
You'll need this
www.whitworths.com.au/azbond-c-contact-adhesive-for-pvc-or-polyurethane-250ml
and this
www.bunnings.com.au/protek-250ml-clear-priming-fluid_p4750123
(methyl ethyl ketone)and YouTube.

Also, search "inflatable SUP repair" using the search button above.

I had a wee scratch pin hole in a 14' iSUP that holds 20psi. Recommendation from a yachty type friend used to dealing with the problem in inflatable dinghies was to use a drop of super glue along the scratch. Still going strong after a year and it was an instant fix too!

PVC adhesive did not hold-eventually air bubbles formed under pressure. I talked with local SUP hire owner who says he has same problems. Recommended WakeNSURF on the Tweed - they fix using ultrasonic technology
( no adhesives) - I have had no problems since.