You'll have to bung or drain the system. If you remove the plate from the valve you can sometimes free it off, or dismantle and clean.
Or you can buy another plate/ball moving bit without the head, ie lower part of pic:
Part 12612 or 3 there.
http://www.bes.co.uk/products/112.asp
2 port and 3 port plate/ball parts are identical
Or you can buy another plate/ball moving bit without the head, ie lower part of pic:
http://www.bes.co.uk/products/112.asp
2 port and 3 port plate/ball parts are identical
