Where do I take the feed point to the antenna from? I realise I don't
have a hope in hell of matching 140 ohms without an "L network" or
whatever, but is that really necessary? Could I get away with taking
the antenna feed direct from a tap off the tank coil?

Paul-

You could use a tap, or add a winding to the coil. If it could be assumed that
the antenna was 50 Ohms, the impedance ratio needed would appear to be 140/50,
or 2.8. Therefore turns ratio would be 2.8 * 2.8, or almost 8:1. In other
words, if the coil had 8 turns, the 50 Ohm tap would be at one turn.

With a second winding, the capacitor would not be needed to block DC. However,
it would be a good idea to include a variable capacitor as Clive suggested, to
tune out any series reactance.

Remember that the telescopic whip is only one half of the antenna system. You
need some kind of counterpoise (ground) to balance it.

73, Fred, K4DII