Gift from US in UK customs

My sister posted my children a present (of a few items of clothing) back in January from the US, by standard airmail post.
We have still not received it.
She wrote 'new clothing' on the customs form.
I am wondering whether it is now stuck in customs, waiting for us to pay VAT?
I have had no notice of what's happened.
Would I be sent a letter informing me of the charges, if that was the case?
It would be such a shame for my children not to receive this present from their cousins, who picked out the clothing items for them.
At this point, i'm resigned to the fact it's been lost or stolen en route, but am trying this forum as a last resort.
Many thanks.

Hi If the package was sent by post it should end up with the UK postal service. If they had received the package from the US you should have been notified if charges were due. If this happened and the charges were not paid the post office would return the package. Goods are not physically held by customs but by the postal service.

Thanks very much for clarifying - really helpful. I've not received any notification from Royal Mail about charges nor has my sister received it back, so I now guess the parcel went astray in the post or the items were stolen. I'll contact Royal Mail to see if they can help. Many thanks.

Hi estead01Steadman
You’ll be contacted by Royal Mail, Parcelforce or the courier company if you need to pay any VAT, duty or delivery charges (‘handling fees’) to receive your goods.
They’ll send you a bill stating exactly which fees you need to pay. 
They’ll normally hold your parcel for about 3 weeks. 
If you have not paid the bill by then, your parcel will be returned to the sender. 
If you have not heard anything from them, please find out who is handing the package and make enquiries.
Thank you.