Apologies for my multiple postings, but I've found this: https://bugs.launchpad.net/ubuntu/+source/plymouth/+bug/713088
The photo in there is exactly what I get except with the Trisquel green instead of the purple.
I'll probably only get back to this issue nearer to the weekend. So in the meanwhile, if anyone is experiencing this, or has a solution, or has found something useful on the internet, please post here.
