I contacted Sumitomo for their advice and they informed me:
Hello David
Re, your recent request regarding part off blades / inserts
99% of manufacturers parting is captive, which means only their inserts will fit their blades.
Sumitomo are currently offering a package of holders and inserts.
Please contact Paul at STS Tooling (Dublin) for more details
Regards
Andy
So, anyone with a similar dilemma now know the answer.
Looks like I will have to dig deep in the pocket for a blade to take GTN 3 inserts. There seem to be several manufacturers for these inserts, which would suggest that Sumitomo want to keep their system all to themselves.
Dave
The Emerald Isle