^^^ There are some interesting (but inaccurate) guesses above!
A well-drafted will should always state what must happen to something, which would have gone to a particular beneficiary, if the relevant beneficiary should predecease you. (For example, if the main beneficiary of my own will dies before me, I have stipulated that the money which he would have received must be placed into trust for the benefit of his two children).
If no such stipulation exists, where the gift would have gone to a child of the testator (but that child then predeceases the testator), the gift is shared equally between his/her children (if any).
Where a deceased child did not have children of their own, or where (as in your case) the gift was intended for someone other than a child of the testator, the gift falls into the 'residuary estate' of the testator. e.g. if a will gives £1000 each to Fred, Joe and Mary, with the remainder of the estate going to Sue, Fred's death would see his £1000 become part of 'the remainder', so it would thus go to Sue.
If there is no 'residual beneficiary' though (as in the example you've laid before us), a 'partial intestacy' is created. The gift which should have gone to a beneficiary who predeceases the testator is then distributed in accordance with the rules that apply to intestacy.
All of which means that you need to make it perfectly clear in your will as to who should receive your gift if an intended beneficiary predeceases you. Any solicitor or will writer who doesn't suggest that you need to include such a provision in relation to each beneficiary simply shouldn't be in business!