"the 416c is custom in the fact it has been upgraded not only once but twice"
That isn't as good as it sounds lol, you say you paid £1000 for it but they've retailed at £550 - £600 for the past few years brand new. On top of all of that, you'll be far better off listing things separately. It's unlikely somebody will want the replica and all the, respectfully, tat that goes with it.
And as ButcherBill said, you're asking £1.2k but you can't be bothered to list everything done to the gun, what's included etc? Peak laziness.
Nobody will drop a grand on a gun that you promise has been upgraded without a spec sheet.