Welcome to the forum. Although you can post in any forum, the USA forum is here in case of local regs or laws
Do you mean Polyorc? Dark greyish, brittle cold water service?
If you possibly can, get it all out right back to the stopcock.
If you have to get onto it, you can buy a 15mm compression x 1/2 polyorc and I think there is also a 3/8 version. Good luck finding one on the shelf though - we stopped stocking them some years ago. There is someone selling them here.
We recommend City Plumbing Supplies, BES, and Plumbing Superstore for all plumbing supplies.