Cheers all, some good suggestions there, will follow up leads on Tuesday....and report back.
Chewy - There is a lot of space, but (and in partial response to PD) there is no possible way of getting a hatch above (definately - we have looked at every possible way)..... Currently you can climb /upper body through a side hatch into the bilge and operate the seacock, but not ideal /would like to do it from the helms position.....BTW it is /would be just deep enough to lie in down in there, once you've made it under the prop shaft
I have seen some US sites /systems with 'fail shut'......
The other idea I had was (working on the basis that the main concern /risk etc is failure of the joint /pipe where the flexi pipe joins onto the sea cock body - if the skin fitting failed then there would be nothing you can do in any case) to have a solid pipe which mechanically screws to the seacock (so little /no risk of failure) and which then remains solid until it gets to an easily accesible /much higher point (alongside the engine above deck level) where it could then have a valve inline (if required) before going to the flexi-pipe and on to the filter..... But working on the basis that the sponsons will always be above water level, then the water inside the pipe should never be higher than the solid pipe......would still have the manual op seacock at low level /as is.....and no worries about leaving it on a Mooring.....
any thoughts?