GBP10,000 works out at approximately a A$20,000. I would surprised if you could not get at least that in Australia. My original 'guess' was a minimum in the range of $15,000 to $20,000. I am not up to date on the current market though. If you looked at the usedguns website you would see most would be asking for more than that. What they sell for though might be different. For the double rifles that are listed. Vintage rifles are only occasionally listed though.

Keep the rifle for its history though. As that can't be replaed.