A 100% silk shirt fitted tailored shirt may be a bit hard to find, maybe refine your search to cotton silk blend, and you may have better luck.
Try these sites:
http://www.nextag.co.uk/silk-shirts/zzukzB1z41--shop-html
http://www.ciao.co.uk/Fashion_Accessories_5302023_2-colour-gray,material-cotton,fashion_for-women
http://fashionshop.telegraph.co.uk/category_lists_products.php?&supercat=Ladies%20Clothing&catgroup=SHIRTS%20AND%20BLOUSES&sc=Ladies%20shirts%20-%20formal
http://www.artigiano.co.uk/CLOTHING/SHIRTS/icat/acloshi?extcam=ppc_goog_artG_blouses_shirts_ladies_silk_shirt_B&utm_source=google&utm_medium=cpc&utm_term=ladies%20silk%20shirt&utm_campaign=Blouses%20%26%20Shirts
Hope these help.