What are the restrictions or complications around inheriting an ISA if I already have one?
Is it simply that I can’t pay into it or more problematic because of inheritance tax?
If you inherit an ISA from a spouse or civil partner, you can essentially keep the ISA without it affecting your own allowance. No inheritance tax is payable, as with all bequests from spouses/civil partners.
If you inherit an ISA from a parent or anyone else, it stops being an ISA and you just inherit the contents of the ISA (cash or shares). It's subject to the normal inheritance tax of 40% above a certain threshold.