I have a problem with the center board sliding back up the slot as the boat accelerates, what do you chaps recomend for causing friction to avoid this happening?

Short bit of hosepipe screwed to top of board...I need to add one myself actually. Very annoying isnt it?!

is there a pic of that anywhere..or can you explain in a bit more detail?

Find a short length (2-3" will do) of hosepipe or similar tube of stiff rubber that is slightly narrower than your centerboard case. Then screw it to the top of the board, so that as the screws are tightened the tube is squashed and widens thus acting like a brake when in the case. You can even adjust the screws whilst on the water to fine tune the amount of friction.

I quite like a bit of shock cord looped over the handle in heavy weather - it provides some resistance and stops the board coming up accidentaly but if you hit the bottom it doesnt break anything!
Most centre board cases are not exactly parallel so friction devices alone don't always work.

Tom,

I took these for you earlier.
http://www.jonembrey.co.uk/

The only time you really really want the board to stay down is when it is really honking and you are both sat on the corner of the transom hoping the gust will end before you hit the mud at Gerston or wherever.  A friction device almost always holds, but in these circumstances what you want to do is put the board about half-way down and then leave it to attend to other things, like keeping the hull underneath the mast and hanging over the edge and praying.  A loop of shock-cord to drop over the board handle can ease the mind immensely.  I have had in the past a sophisticated system with blocks and cleats attached to the board handle.  These remained attached to same when we hit the spit of Beaulieu River going flat out on a 2-sail reach, sadly the C/B handle parted company with the board, so do make sure there is either a bit of shock cord or some other safety device.

cleats which release on impact are ideal as they do what it says on the tin and give you adjustment which is non stretchy. I use them on centerboard and lifting rudder when in shallow water