I need to take a new feed from my cold water cistern in the loft in order to
hook up a power shower so I've bought a tank connector from Wickes. It
didn't come with any washers, which I assume I need. Is this correct? If so,
what sort - they sell all kinds, leather, rubber etc (I am still talking
about Wickes here by the way). I assume I need to use PTFE on the thread of
the connector. Is it also worth using some kind of sealant (silicone,
plumbers mait, etc) to ensure a bombproof seal?

Also, as it's only a 15mm connector (it will only be used for the shower), I
haven't seen a holesaw that will make a small enough hole. The tank is
plastic - will a flat wood drill bit work, or will it result in me having to
buy a new cistern?

TIA,
Andy