HERE IS THE CORRECT PART. I bought the same shower fitting and read this thread. A little light rummaging on ebay however I came across DOWTY seals, from the hydraulic world. Cheap as chips and actually meant to do this job. They are a rubber insert inside a washer. You just tighten them up with moderate force, and they never leak. PERFECT. I paid £2.59 for 10 1/2 BSP DOWTY seals, also known as bonded washers, delivered to my door. Hope this helps, and the shower mixer itself is utterly fabulous.