Hi Frank S,
If you are UK tax resident and domicile when the pension is withdrawn, you are liable on your worldwide income.
You would need to check the double taxation relief of the country in which the pension is being paid from to see if in fact it is tax free in the UK and doesnt need to be declared.
Please see guidance here:
Tax treaties.
As this forum is only for general advise, you may want to contact us direct on 0300 200 3310 for more specific details.
Thank you.