I'd say you're looking around £500 -£550.
£325 ish for the 416 with the original rail - the new rail is likely to put people off as much as it is draw them to it. Also it looks like a longer barrel and rail than standard (assuming same outer barrel, but with the extension and a longer rail). The fact it's had work done, spectre, spring etc puts as many off as it does attract again, so little value is added in my opinion. Would have priced it lower without the work done. £280-300 ish.
£180 for the pmags - £45 each seems to be around the going rate for them.
£50 for the rahg rail.
I'd probably advise selling the rahg rail separately if you want max cash but more posting to do. Selling the original rail separately isn't easy and doesn't get as much as the rahg as its much more common.