Wharfedale speakers of that era had odd diameters, especially the W15 which was actually a 13 inch speaker. The effective cone diameter was a little larger tha the avr\erage 12 incher and less than that of an average 15 incher.
That being said, attached is a cardboard ring I use for refoaming Wharfedale 12 inchers. The inside diameter is 240mm and the outside diameter is 296mm. I would recommend using a felt (CS = cloth surround) as it is easier at get. However, Joann's fabric shops have both felt and foam. The foam, unfortunately is a little too thick, about 5.2mm whereas the original foam was about 2mm thick. The thicker foam I have also has a thin cloth backing which is easily removed and should be as this will affect the elasticity of the foam.
Now, to apply the annulus (surround), the voice coil should be shimmed to lock it in position which requires removing the dustcap. The proper height of the coil should be when the spider is flat. If this unit has a phenolic spider, you're in luck as the whole cone and coil assembly is then easily removed.
To remove the dustcap, hopefully the adhesive has deteriorated in time and it will lift easily. If you have to use acetone, use very little. Too much will allow the acetone to get to the apex of the cone and may weaken the coil to cone attachment.
Plastic shim stock should be available from hobby centers. Paper can be used, preferably that which is thicker than 20 pound bond. Inkjet photo paper might work well. The initial shim should be cut about about an inch wide and about 5 inches in length, which will be about a quarter inch shorter than the inside circumference of the coil. It should be thick enough to insert easily and apply a little friction to the moving system. Thinner pieces can be inserted to lock the coil. This locking doesn't have to be rigid; just stiff enough to hold in place while attaching the annulus. If the cone and coil move up or down during this process, it can be brought back to median rest position (flat spider). This is only important when applying the annulus to the frame. The main purpose of the shims is to keep the coil parallel to the pole piece, assuming the spider glue is still holding.
White wood glue is recommends as it takes time to set and mispositioning the annulus can be corrected easily. Let the glue set to the cone before working on the annulus to basket. When dong so, keep the spider flat as well as the annulus. Let the glue set to the point at which it holds on its own, not necessarily dry before applying the green felt gasket. Once the gasket is applied, place a sheet of plywood on it and add a few books for weight. If you're manually dexterous enough, you can flip the speaker with the plywood. Alternatively, you can mark on the plywood, the mounting holes for the speaker, drill them and use machine screws to press it down. With the speaker inverted, you can check the cone to annulus adhesion and apply more glue, best done with a small artist's brush.
Once all is done, don't forget to remove the shims before applying the dustcap. This may sound funny but I've fallen into that one a few times years ago. Prior to that, I placed a small can of grease in a car engine cylinder, just because it fit so nicely and didn't notice it prior to installing the heads. Needless to say, it came very abruptly to my attention when the starter was engaged. :lmao:
There may be some useful info on these pages in my website.
http://www.ln271828.net/wharfedale.html
OOPS. The acquisition of a speaker repair kit may be facilitated now that you have the dimensions of the surround. I believe Parts Express carries them.
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Don't tell anybody