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I was going to suggest less but my ebay trawl of 'nice' mixtes suggests otherwise:
Nice Mercian sold for £695
Ratty Mercian sold for £400
http://www.ebay.co.uk/itm/Mercian-Reynolds-653-Mixte-Road-Bike-Dura-Ace-XT-DT-Swiss-/281797260866?hash=item419c6cf642%3Ag%3AbSgAAOSwWnFV8vmV&nma=true&si=Q7DtohU5BB3zfQQCD2tXPGaf3zw%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557Given the unkown builder I'd suggest £4-500 is a good bet.
But mixtes aren't a limited market so they may not find much interest.
I would go higher than Ed - its easily a £400 bike IMO