Thanks for all the advice, I'll ask Lexus Croydon if they can download a later gearbox ECU if available. (I presume they are upgradable rather than having to fork out for a new chip). I'll post an update if they do.
I'll also consider replacing the plugs becuase I'm still not entirely conviced that it is all down to the gearbox ECU. I do seem to remember that the car ran a lot better following one of the services (plug change), although this didn't last long..
In answer to your question Colin, yes I do believe it was a demonstrator originally at the Guildford dealership.