I cannot quite picture your arrangement but if you are connecting your flexible tap connector directly on to the isolator valve then I am not surprised you are experiencing leakage problems. The isolator has connections that involve the use of an olive to provide the water tight seal the end of the thread has a chamfer on the inside to mate with an olive whilst the very end does not have a flat surface to mate with anything tending to be rather sharp too.

What I suspect you need is something like these:

https://www.screwfix.com/p/tesla-bra...x-2-pack/2665r

The plain end goes in your compression fitting on the isolator with an olive off course and your tap connector on the threaded end with the rubber washer you mentioned initially.

A link to a photo of what you have got would help

Richard