Hi Gavin,
Article 17 provides for the taxation of pensions and other similar remuneration only in the state of residence of the beneficial owner.
For this purpose, a payment is treated as a pension or other similar remuneration if it is a payment under a pension scheme, as defined at Article 3(1)(o).
Contrary to this general rule, the residence state, under paragraph 1(b), must exempt from tax any amount of such pensions or other similar remuneration that would be exempt from tax in the State in which the pension scheme is established if the recipient were a resident of that State. Thus, for example, a distribution from a US Individual Retirement Arrangement or ""IRA"" to a UK resident will be exempt from tax in the UK to the same extent that the distribution would be exempt from tax in the US.
(
DT19853 - Double Taxation Relief Manual: Guidance by country: United States of America: Notes).
Thank you.