Need advice on who to get advice from.......?

Hoenir

Intrugued to know what's in the collection to make it so valuable? 

Keep_pedalling

probate_slave said:

Yes, certainly nothing for you or your sister to declare on your tax return, and you're free to bank the gift.
But note the possibility that CGT may be due on your mother if the model car collection was valued on your father's death and achieves significantly more at auction.

CGT on personal possessions does not apply to individual items under £6k, and as this is a collection of model cars I would imaging few is any of them would exceed that value.

probate_slave

Well, £50-60k was mentioned in the OP, and I assumed the collection will be sold as a set.