I'm in the process of fitting out an effectively new hull. Everything is done apart from string to keep the spinnaker up. I've got two main questions with regard to how to do it:

1. How far aft of the mast should the turning blocks in the deck for sheet/guy be? 

Is, about half way, to suit where I can get a good lead on blocks ok?

2. Do I need to have separate cleats for the sheet/guy, given the direction of pull? 

It seems that older boats have just one positioned to favour the angle of the sheet, I would have thought if you were going to have only one, it would positioned to favour the guy. That all said, not sure how I would position it to favour the guy, maybe thats why they are the way they are. Or have I got it totally wrong?

All suggestions gratefully received

Whilst on most spinnaker boats the turning blocks are as far aft as possible (look at a fireball, or scorpion) through-deck turning blocks in merlins are usually 
1. at the widest point of the hull and 
2. just aft of the crews normal upwind hiked position

Some newer wooden boats have previously exploited the fact that the width remains pretty constant going aft, and have moved the lead position as far back as the width remains constant (3400 - Dangerbat was an extreme example)but this is unusual.

If your merlin is an older narrow sterned type (NSM1 / 2 etc) then put the sheeting 'holes' just aft of the thwart and cross 'rib'.

Cleats. 
Most boats cope with just one cleat on each side, with the sheet running from the deck down to a swivel (ratchet) block usually on the thwart, and led so that it can be cleated from the block when hiked on the opposite side.  Some boats place another cleat on each side on the inside of the side-deck (on the Carlin) so that the crew (if unusually strong)can adjust the guy position upwards when hiked.  

These are very unpopular with crews in my experience, as 
a)if they are of the 'slighter' persuasion they aren't strong enough to use them, and 
b)they are perfectly located to bruise the calf muscles, hips, arms etc.
I removed them from my NSM1.

Some boats have a second cleat, cunningly arranged between the thwart turning block and the side-deck through block which will only engage the sheet / guy when it is being pulled from above (i.e. when adjusting the guy) - when pulled from the opposite side, the turning block flops over and away from the side of the boat, putting the cleat out of reach, so that it doesn't interfere with playing the sheet - very clever, but again, it only works if the crew is strong enough to pull the guy from above on a windy reach.

Once your cleats are set, you'll have to consider whether you have knots in your sheets or just marks.  Knots are good for weaker crews as they are set so that the guy can't go out any further than it would if set for a tight reach.

Again stronger crews frequently don't have knots and set the sheet and guy to a mark instead.  It can make the pole quicker to set up.

Here's a pic that shows where the turning blocks go. Not ideal, but better than nothing.

Thanks for a wonderfully complete answer; I have a plan.

Can anybody suggest length / diameter for spinnaker sheets and halyard please?

I use 4mm for the halyard, sheets are tapered 6mm to 4mm I think.  You might find an anser in the link below..... or maybe not