altering a neckline.

I agree with3xnanny that shirring elastic will only work with knitted fabric.I've done that with a few tops which have stretched.
Another idea is to make a 'dickey' - a sort of false blouse front.
But that would change the style altogether.

I'm glad that others have found White Stuff necklines difficult.

And, in fact, despite the price, some of their goods are not good quality.

I've also added pin tucks in the centre, perhaps 6 in all carefully spaced which seems to work fairly well.

I'm glad that others have found White Stuff necklines difficult.
I thought it was just me.

Recently I altered the neckline of one of their tops, by lifting the shoulder seams, and I have used both the elastic, and the pin tuck ideas,on other clothes.

The pants would probably turn out better if tapered all the way from crotch to hem. The crotch alteration would be minimal, but the actual seam for the alteration would increase as you moved down the leg. By starting at the knee you might easily find you had created a nice pair of jodhpurs!

You could use lingerie elastic the one with the slight picot edging around the neckline to bring it in, but again it depends by how much. You could sew a bias channel inside the neckline and thread some 4mm flat elastic through and gather it up till it fits. Sew a small ribbon bow at the centre front and it will look like one of those peasant blouse necklines.

I think the shirring elastic idea works well if it is a knitted fabric, but unlikely to look right if it is not - darts are much better.
Don't cut any fabric away on either garment until you have tacked it and tried on.

I've done the pin tuck idea, too. I think it would look better than the shirring elastic. Funny that someone mentioned White Stuff. I had two of their dresses a couple of years ago and found the necklines far too wide so I took pin tucks in the back of the neck. It worked well.

I've taken pintucks in tops that have this problem. Three or five at the centre of the neckline.

I've done the same as Illte and did add some very tiny buttons where I pleated the neckline.
I put the pleats each side, off centre so it looks as if that's the design, above the bust line.
In fact, I have another one the same style to do, this thread has prompted me.

I have a cotton top two sizes too wide, and I put two large darts in the lower third of it to make it a cocoon shape. Very successful.

Or you could add a dart from the neck to an inch or so above each nipple. Put the top on inside out and pin them so that it fits better, turn it right way in and try it on again to check it looks right, even them up to match one another and sew them.

If you narrow thelower part of trousers, don't take all the extra off at the outsides of the legs, or it'll look odd (like jodhpurs) . Be guided by another pair that are the shape you want.

I've taken pintucks in tops that have this problem. Three or five at the centre of the neckline. White Stuff often have this style.

Ive also lifted tops at the shoulder to bring the neckline up.

It really depends on the style of the original top.

Without seeing the top it's difficult to know what to suggest, but threading shirring elastic sounds a good plan - worth a try and if it doesn't work you can just remove it.

I have tapered trousers in from the knee, using a narrower pair as a guide - don't just eyeball it, as you could end up with them being too tight. If possible, just baste them and then try them on before trimming the excess fabric off. If OK, then stitch, trim and finish the raw edges properly.