Hi Oldhippyone
If Scotland's Registration system is the same as England and Wales, then it would also be possible to register an overseas death in Scotland as well as in Tenerife, but, when my father died in Spain, we didn't bother doing it, because we saw no point at the time and is also cost an extra £70 as far as I remember.
I had another response from a Registry office today and Margaret's death was not registered there as far as they can tell, but they are looking again. Since they emailed me, I was able this time to ask them if they have any suggestions as to which Registry office to try next, otherwise it's a hit and miss thing, so I'm hoping they may be a bot more understanding and helpful than they were at the Santa Cruz Registry Office.
I tried contacting one crematorium I thought that might be a possible, but haven't had any response sadly to my communications. I know you say the sisters traveled a long way on what was like a motorway to collect Margaret's ashes and I know it's difficult to remember with the shock at the time, but do they have any idea about how long they traveled for - 1 hour, 2 hours 3 hours etc; which could help narrow down the crematorium at least and then I cam try and contact them too.