Anyone out there with shares in Anglo American SIP? What effect Mondi demerger will have, rate of tax & NI?

Answer 1

A SIPP is a Self Invested Pension Plan = however (as far as I can tell) there is no such thing as an "Anglo American SIP" (or even a "Anglo American SIPP :-) )

Since the rest of your question relates to financial matters, I assume you have some sort of investment in Anglo American .. in which case you should already have been informed of the terms of the demerger.

It will have no effect on your Tax situation, unless :-
1) you hold the shares outside a SIPP or ISA
AND
2) you sell and the TOTAL profit from ALL share sales during the Tax year exceeds your Capital Gains allowance (about £8,000)

I've no idea why you mention NI (National Insurance) since NI is paid on your wages (only).