Also, what sources have you checked? It should be possible to source the single write variant online without problem.
Thanks heaps for your prompt reply!
I've looked at Mouser and Digi-Key and they have a $60 minimum for free postage (I don't want to order 100 for free shipping at this stage obviously as I'm only prototyping) and I can't bring myself to spending $25 on shipping for around $5.00 of modules .
Which leaves me with my normal supplier of Element14 (formally Farnell) where I get free shipping regardless of order quantity. but they don't source the Single-Wire ones at all.
I was going to put a post over on the OTA Flash forum as well as I'm having issues sourcing the AT25DF512C (it's been discontinued) and was looking at an ATAES132A (Datasheet <here> using an SPI interface) as I accidentally ordered some of them for another project and thought maybe it would be possible to combine both the security signing and OTA flash in the one component (Which is a HUGE assumption as to its use). As mentioned earlier, I'm still fairly new to the engineering side of hardware so I may be way off the mark.