The Anatomy of the Ship book 'The Battleship HMS Warspite' uses the name 'deck pipe' for the pipe that goes down to the cable locker (page 109). On top of these pipes there is a cover which resembles a 90 degree pipe bend. They are located close to the capstans.
(The other end of the chains go through the hawse pipes, from the deck down and out through the ships sides).

I assume that you are asking about shaping the deck pipes ?
I checked the sprues and instructions in this review:
http://www.modelwarships.com/reviews/ships/hms/bb/warspite-350-acy/acy-review.html
and I get the impression that these items are missing completely. Is that correct?

In the anatomy book about HMS Hood these pipes are called navel pipes and they are provided with a "water tight bonnet". Since the Hood was designed and built only a few years after the Warspite (laid down 1916 versus launched 1913) I think it would be safe to assume that the "bonnets" on Warspites navel pipes would also be watertight, i.e. we don't need to worry about shaping the insides of the "megaphone". These covers need to be watertight to avoid getting a lot of ocean into the chain/cable lockers when waves break over the forecastle. The hawse pipes also have covers but those are mainly to prevent accidents when working on the forecastle deck.
Making the bolts that hold down the bonnet might be possible in 1/350th but maybe you can skip these. If needed, drill holes with a 0.3 mm drill and insert short bits of stretched sprue, file/polish to suitable height when it has dried. These will be oversize, 0.3 mm will be close to 4 inch bolt heads in 1:1 scale so using a thinner stretched sprue and gluing with CA could be possible, apply glue from below. The capillary action will draw the glue in and make it look like a washer under the bolt head. Don't spend time on this unless you feel that it is needed.
Making the bonnet: you will need a circular disk (punch and die set available ?)a 90 degree bend and a U shaped flat piece. Try to find three 90 degree bends of suitable diameter in some sprue. Otherwise it is time for sprue stretching to get a suitable diameter. When these have cooled down it is time to heat them gently in one spot in the middle so that you can make a 90 degree bend.
Cut out the bend and file to shape. The opening where the chain enters is not a circle, it is more like an upside down U. This shape and the strenghtening lip around it (see your image, page 110 from the anatomy book ??) can be made by gluing a piece of styrene sheet to the "opening" and filing/sanding it to shape when the glue has dried.

One way to get rid of the sharp 90 degree angles where the "pipe" part of the bonnet attaches to the bolting flange and the watertight lid (the U shape) is to use gloss Humbrol enamel (Ivory white in particular), this paint has the irritating habit of not staying on a flat surface if there is a nice and cosy corner to crawl into, this creates a smooth radius instead of the sharp angles. Other types of gloss paint might also be possible to use,try on something else first.

Summary.
Three parts needed, flat disc, bend from streched sprue and a U-shaped lid (facing the chain coming from the capstan).

/ Robin