An incorrect model was fed into the program. The feedline actually radiates
like the clappers and the whole thing behaves like an inverted-L with a
small 3 metres spur which does very little.

If the severe off-centre feedpoint MUST be used, probably the best way to
feed the antenna is via an open-wire line and just accept whatever impedance
is presented to the tuner, via a good choke, at the transmitter end.

As the antenna will have the crude general characteristics of a long, random
length, inverted-L, why not go the whole way - erect an inverted-L and feed
it against ground? Easily modelled and tuned. It also becomes multi-band.
You won't need a particularly good ground connection.
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Reg, G4FGQ