Think I might have sorted it… (and I’m a tad embarrassed too 
). I had a nose inside to check some details for the case and I noticed the cable from the psu to the drive was obstructing a fan. In my haste to get the SSD up and running I’d not tidied the cables properly. Since clearing that up, things seem to have been ok, it’s been up for over 3 hours with no problems at all. I guess the hard disks can cope with a hotter temperature which is why they still worked. Certainly explains why the problems got worse over time when my pc was left on.
If people need to find out the temps of the hard drives then a good app is SpeedFan, my ssd is now running at a nice 27 degrees!
Oh and i might add, i got a fairly swift response from Sandisk with loads of detailed questions which was a very good response so it’s definitely worth raising a case if you have issues 
