Ok sorry if this sounds very basic to most of you. Finally started to rig boat (Rapidity), however completely new to Merlins, got the mast upright seated in original position in metal track? on floor of boat, attached side stays, raised jib to get front tension and the mast sits in middle of gate with a gap in front and behind it. Is this normal or should I be using some sort of mast chocks ?? Then when we tried to get Jib lowered the wire halyard where its joined to rope jammed at bottom pulley and no way would it go back up the mast, any ideas, or am I going to have to replace it ??

The mast gate should hold the mast securely so it cannot wobble sideways, but it will allow it to move fore/aft. You'll see it bend as you apply rig tension.

Typically, you'd have a mast ram (big lever on foredeck) which controls the position of the mast just above the gate. Or maybe a screw arrangment that pushes on it. If not then perhaps you have 'lowers' (short lower shrouds)?

The wire/rope splice can be a bit of a pain to shove back through the pulley. If it fitted before, it can fit again, surely!

Obviously it's a little difficult to be precise without seeing the set-up of your particular boat (Merlins are all different) but here are a few thoughts. It sounds as if you have correctly seated the mastfoot tenon into the mastfoot on the hog and also that you have correctly attached the shrouds at each side of the boat. It also sounds as if you have correctly hoisted the jib on the jib halyard which is likely to be a wire halyard with a rope tail attached to a soft eye on the end of the wire section. In this case, yes it is very common for the mast to sit roughly in the middle of the mast gate (at deck level) without touching either the front or back. Often there is a thin metal strip which can close off the back of the mast gate but this is only to stop the mast falling out of the boat as you lower the jib (given that few Merlins have a forestay). The extra space in the mast gate can allow variations in mast bend. This bend can be controlled by a mast strut, mast ram or various pulley operated rope systems (or by lower shrouds in newer boats). Mast chocks are not common on Merlins, but any system is possible. Some older Merlins had masts that were so stiff they used very little control over mast bend. With regard to the jib halyard not allowing you to drop the jib, I presume that the halyard passes through a sheave system at the foot of the mast and I presume you managed to get the rope tail and the (hopefully) soft eye through the sheave system in the first place in order to hoist the jib. The soft eye would normally be a few inches long and very flexible. The only impediment to its passing through the sheave should be the ferrule at the head of the soft eye. There should be no thimble in the eye. If this is all correct then a possible cause of your troubles could be the sheave itself. This is fairly easy to replace. You should be able to drift out the metal roll pin which acts as the axle for the sheave(s). You can buy replacement nylon sheaves which are easy to fit and hopefully should rotate freely enough to allow the halyard to pass over them. Clearly if someone has fitted a halyard with incorrect fittings then my suggestions are all a waste of time. Apologies if this is not what you were looking for!!

Many thanks for the comments guys. Yes, thats exactly the situation, I will have to have a closer look to see what is causing the jam.

As I recall from a long time ago (boat no 2749 when new) we used a combination of(loose) plywood spacers (sort of t-shirt shape so they stay in place by gravity) in front of the mast to limit mast bend.  Easy DIY.  Apart from limiting mast bend/ stiffening the rig they also help spread the load onto the mast as the front of the gate is probably somewhat of a point loading. Might be what you need?

If you have an old Proctor D the halyard sheaves were Tufnol IIRC and after a few years will wear on the edges enough for the wire to jump off the sheave and jam - bet that is what has happened.  You need an alloy or s/s sheave with a wire halyard.

Thanks again guys. On inspection of my Jib Halyard I found that the wire soft loop and ferralls were passing through roller at foot of mast ok, but the rope where it was whipped was stiff and jamming. WD40 sprayed onto it helped considerably. It also seemed much improved after getting wet !