1918-P Half Sovereign George V MS

1918 half sovereign mounted in 22 carat gold with sympathetic flat claws .... does having been mounted detract from the value of said coin?

Yes, such mounting would reduce the value to a small fraction of what it would normally be worth.

1918-P Half Sovereign George V MS	25-Jul-2010
1940 Florin George VI MS	14-Jul-2010
1913 Florin George V MS	14-Jul-2010
1938 Florin George VI MS	15-Jun-2010
1885-M Half Sovereign Young Head MS	11-Jun-2010
penny sets	13-May-2010
Pre decimal coins available	04-Apr-2010
1872-M Medal Die Alignment Full Sovereign Shield Reverse SP	11-Mar-2010
1954 Threepence Elizabeth II MS	02-Mar-2010
1914-M Full Sovereign George V - Large Head MS	17-Feb-2010
1931 Penny George V MS	02-Feb-2010
1948 Penny George VI MS	18-Oct-2009