Scoot's contract of carriage can be found here: https://cdn.flyscoot.com/prod/docs/default-source/doc-coc/scoot_conditions_of_carriage_final.pdf
While refunds are generally not given, 14.9 does provide for a refund in the event 'previously confirmed space is no longer available'.
You were booked in ScootBiz and for your flight, this confirmed space was no longer available.
You would therefore be entitled to a refund under 14.10.
I note via flightaware that Scoot has been operating an A320 on at least some flights recently. If this was the case, there would be no ScootBiz cabain, only economy.
If however they chartered a Singapore Airlines A330, you would have been seated in the Singapore business class cabin - the equivalent of ScootBiz in all respects (but actually a much better seat in your favour).
If the flight was indeed operated by an A330 you would not be entitled to a refund under Scoot's terms and conditions.
I don't have access to flightaware or flaightradar to check the aircraft that operated on 5 May. This might be the key to working out your entitlements.
(You don't appear to be claiming the cost of your Singapore airlines business class ticket which you used as a replacement. This would likely be outside any refund entitlement.)
Thank you MEL_Traveller for your detailed and well informed response - greatly appreciated. I was told twice during telephone calls to Scoot in Singapore that the replacement aircraft was in 'all economy' configuration and that I would not have a business class seat.
Any ideas about claiming against 14.9 ??? Following my unsuccessful telephone request for a refund I sent a registered letter to the CEO of Scoot - but no reply. Maybe ASIC ??