Brilliant, thank you for your help. Now I just need to workout how to suspend a big inflatable ball covered in wet glue in my house without accidentally ruining a cat.

Thanks again

I don't think attempting to PM over a partly-inflated ball will work:  Every time you touch it or move it, it's going to change shape. 

Forget the cling film -- plastic is plastic, and not much sticks to it, and the extra layer will just complicate your problems.

The only way I can see to do this is to fully inflate the ball and PM with paper all over it.  Don't try to do the entire ball in one go -- do half, let it dry enough so it doesn't peel off.  One problem you'll discover as you go is that wet PM is heavy -- the part you have covered will keep trying to roll to the bottom.  Sit the vinyl ball in a large bowl or tub (etc) to keep it from rolling around. 

Do the top half in one or two layers, and then aim a fan at it if you can, to dry it.  Then rotate the ball so the covered side is down and do the other half.  Don't cover the valve thing, leave a 15 cm (+-) opening so you can remove the ball later. 

David is right, use the stronger kraft paper, if possible, and two colors of it would be ideal (it comes in brown and white).  It would take you forever with newspaper.  Make each complete layer in a different color so you will be able to cover it evenly.  Start with three layers of kraft paper and let it dry, then let a little of the air out of the ball and press on the PM to see if it's strong enough to hold its shape.  If it flexes too much, and it may for something that large, add more layers until it's firm enough to suit you.  You might have to re-inflate the ball if the paper shell is too floppy.  BUT DON'T OVER-INFLATE or any weak joints may pull apart.

You always see people applying paper strips, but on round things, triangles often lie down more smoothly.  Lay your paper on a table, hold a wooden or stiff metal ruler (or stick) on it, and tear into pieces.  You'll have to experiment to find the best size -- if the triangles wrinkle too much, make them smaller (just tear them in half).  Perfect triangles aren't necessary, just 3-sided pieces of paper.  The smoother you apply the paper, the smoother it will be when you're finished. 

Drying times will depend on how wet you get the paper and how humid your environment is.  It needs to be wet enough to make the paper more flexible, but not too wet.  Don't set the ball near high heat -- if the air in the ball heats up too much, it will expand.  If the vinyl ball tries to expand too much, it may cause the paper layers to start pulling apart.

When you've got enough layers so you can handle it without it flexing too much, let the air out of the vinyl ball.  Let it contract by itself, don't rush it.  It should pull loose from the inside of your paper shell.  Then reach in and gently loosen any part that is still sticking to the paper, and remove the ball.

Now get what we call a 'box knife' over here across the pond.  One of these: http://www.harborfreight.com/utility-knife-3359.html  ... oh, they call it a 'utility knife', too.  Or a single-edged razor blade, or a very sharp knife.

What I think you'll want to do is cut a shape like an apple slice out of the ball, and let the opposite side flex like a hinge.  I'm sure you'll want to make your cut-out to include the hole where the valve stem was (and it will be a handy place to grip).  Mark it with a felt-tip pen first.  Cut out less than you think you'll want.  You can always cut more away to get it to look right, but cutting too much at once might end in a big 'OOPS'.

David suggested maybe 12 layers, and he may be right.  But I would suggest fewer layers to start, so you can cut it easily.  You want the original shell strong enough to handle and not flex too much while you cut; once you get it cut and shaped as you want it, you can add additional layers for strength and to hold the shape you want.  Save the cut out piece for testing your paints on.

To brace the mouth open, I think you could use (temporary) wide strips of corrugated cardboard with slits cut at top and bottom to hold the 'lips' apart while you're adding more layers of PM.  Cut the cardboard strips so the corrugations run up and down for strength.  You might have to cut the slits at an angle to fit the 'lips' into them.  Or use sticks if you've got them. 

Leave them in place until the main piece is dry enough to hold itself rigid.  Then finish the lips.

Smoothing carefully as you go will really reduce the amount of sanding you have to do.  You should be able to smooth out the finish (before the paint) by brushing on acrylic emulsion paint with some chalk/whiting/calcium carbonate mixed in to make it thicker.  Try it on the waste cut-out first, to make sure you're getting the effect you want, as you may have to make some adjustments as to amounts.  Then you can paint on top of this.