Reclaiming US tax deducted from sale of shares for a UK resident tax payer

I am a UK tax payer, resident in the UK.

I owned some shares in the US, and sold them last year. Due to some paperwork issues, when I sold the shares I had US tax deducted from the proceeds.

I had expected to have this refunded from my UK tax, as I thought a double tax agreement  applies (which it does). - However, as I made capital losses on the sale of shares in the UK, I paid no capital gains tax, and no credit for the US tax is possible.

The share company registrar unhelpfully says it is between me and the US tax authorities.

Is there a way I can easily reclaim the tax. The only suggestion I ave had made to me is that I need to complete a US tax return, but I do not have a US tax reference etc. Any help and advice would be appreciated