I am a German citizen and have lived in the UK for over 15 years. It would appear that I therefore count as UK domiciled.
You may be deemed domiciled for UK IHT if you have been UK resident for at least 17 tax years out of the last 20 tax years.
In the event of their death, I understand (from this forum, e.g. http://www.taxationweb.co.uk/forum/inheriting-foreign-property-from-foreign-national-t31537.html, and elsewhere on the web) that I have to pay inheritance tax in Germany but not in the UK, as the deceased is not domiciled in the UK and the estate is also located outside the UK.
Correct.
Assuming this is still correct, is there any need (1) to seek confirmation of the above from the HMRC in writing or (2) to declare any such inheritance to the HMRC in any way, in order to prevent any future problems (e.g. when transferring some of the funds to the UK)? I am aware that I would need to declare any foreign income if I choose to invest the inheritance abroad.
(1) No; not necessary. Would basically be a waste of time.
(2) No.