The other day, when I was enjoy the opportunity to take a small break from sailing and stand on the centreboard of the Merlin ... I noticed it rakes forward quite a way ... more than 5 degrees I would estimate ... There is a control line that can be cleated such that when at speed it prevents it going back into the case ...

Is it normal to have the centreboard raked forward at low speeds and then, presumably vertical once you get going a bit quicker?  Whats the *ideal* scenario??? (I'm assuming upwind and reaching)

It's supposed to reduce wing-tip turbulence a bit. Have a look at a glider and you will notice the wings are ever so slightly raked forward. It will also move the centre of resistance slightly further forward which may help balance of the boat in lighter winds, as you rake the rig back in more wind you may want to move the plate back slightly to keep the centre of effort near the centre of resistance.

Is it designed to rake forward? If you have an older boat, the board might originally have had a couple of large stoppers on the top to grab hold of. If those have been changed in preference for a short piece of rope, you might find the top of the board goes lower, allowing it to rake further forward than designed.
Is it balanced when raked forward? Whether it was part of the original design or not, but you might find the boat is nicely balanced: so that raking forward does no harm. When sailing along on the beat, sails setting properly, boat upright, letting go of the tiller should allow the boat to luff gradually. 
If the boat stays straight or bears away: rake the board forward.
If the boat luffs fairly quickly: rake the board back a bit (or buy yourself a new jib!).

so basically ... it should rake forward at any speed, up until you start raking the mast back. I'm pretty sure I've seen it pop from its "raked forward" position, where it rests agasint the back of the centreboard case .. to its "about vertical" where it is restrained by a bit of string in a cleat, when at speed .. but there again, I'm usually otherwise occupied at that point ... hmmm I'll try and remember to have another look next time we're out.

I don't see any stoppers ... I presume these would have been a couple of bungs, like the sort that are sold as door stops? Its possible that 

I was fairly surprised by the amount of rake .. gould have been 10 degrees, but I was trying to stp the thing turning turtle at the time, so I didn't stop to measure it.  

Good point re balance, I just bought a new(er) jib and got to calibrate the rig, so I'll check the balance after that is all done, 

thats a very good point, cheers.