OK. I can see where you're going wrong.
Leaving aside the extension for a moment, the usual method of coupling to a tap tail is by using a tap connector, viz:
or:
In the first image you can just see the red fibre washer that will seal against the bottom edge of the tap tail (aka tap shank). A flexible hose connector seals the same way, but might have a rubber washer. The John Guest connector (second image) has a rubber washer. If you use any of these types of connectors then
on no account should you be putting PTFE tape on any threads. Not anywhere.
The reason for this is that it will interfere with what would otherwise be the easy tightening up of the connector.
Occasionally, a tap tail has very little 'land' (area of metal) against which the flat fibre (or rubber) washer can seal, but the tail is instead more-than-averagely-tapered on the inside - this type is often better suited to using a connector like the much-maligned Hep2o HD25B (but I believe the Hep ones aren't for sale at the moment). You still shouldn't need to use any PTFE, or paste, or anything extra, on the sealing faces.
Returning to the extension: assuming that you've added this for reasons of access, and assuming that it's just a female-to-male BSP coupling of some kind, then you should seal the bottom end of the extension just as if it were a tap tail. The female connection to the male tap tail, however, might be different. If the female end isn't like a tap connector, then I'd use PTFE tape on the part of the tap tail that will be mated to the extension. The amount of tape depends on the amount of play between the two fittings. But my rule of thumb is to make it difficult to turn the fitting by hand for at least half of the distance that you screw it on.
If any of this is unclear, please ask.
