It’s the Swiss Withholding Tax.
I’m told due to the ‘Double Tax Treaty’ between the UK and Switzerland it may be possible to reclaim 4/7th leaving a tax of 15%. Not entirely sure how to raise the claim but can try and find out.
Worth the hassle if it’s a big amount.
If this is in a SIPP then I’m told you’d need the SIPP administration to raise this which FT / lot of other brokers don’t support.