@WillsandGrace "Parent passed away. Everything goes to other surviving parent. As per Scot's law half of bank account monies after fees etc goes to surviving spouse, the other half gets split between children."
"As far as I'm aware my parents always had separate bank accounts, nothing joint."
This isn't right. Assuming there was a will, then the surviving spouse would have one third and the children/grandchildren would also share one third. The remaining third would be distributed according to the will.
If there wasn't a will, so the parent died intestate, then there are different rules. From what you've said it sounds as though there was a will.
"...I received a lawyer letter around a year after the death, advising me of the amount I could accept or reject"
"I then received a nasty message from one sibling ... as they had to pay for x, y and z ... and would put things into a deficit, which I didn't understand,"
Your sibling has been up to something which, if it isn't dodgy, then is decidedly reckless at least.
"This has led me to believe that they have spent this money after death but prior to will being sorted and if this was allowed, which is what I'm trying to determine."
The legal rights that exist under Scottish law to either one third or one half of the moveable estate are treated as a debt owed by the estate - just like if your parent had an outstanding mortgage or credit card debt when they died. This should be paid off before any money is distributed.
Part of this process is having the children and spouse either "accept" or "reject" their legal rights. That is what the letter from the solicitor was about. If you had rejected your rights then that money would be added back into the pot and distributed according to the will.
It sounds as though your sibling has spent all (or most) of the money after your parent passed away. The solicitor has now been in touch with him saying that he needs to pay you £x which is the amount of your legal rights. Since your sibling has spent all (or most) of the money already they are now required to pay you that money and it sounds as though they don't have it.