I think the starting point is to determine what exactly was the current nature of the ownership of the property immediately prior to your uncle's death.
You state that the ex-wife is
the legal owner of half the house
You do not state whether the house is in England. It is important to realise that different consideration may arise, for example, in Scotland.
In England a property that is jointly owned could be held in one of two ways:
1. as joint owners, or
2. as tenants-in-common
If A & B own a property as joint owners then on the death of either joint owner the other joint owner becomes the sole owner and this occurs by operation of law not by disposition of the deceased. In this situation if your uncle and the ex-wfe were joint owners then the ex-wife would become the sole owner of the property on the death of your uncle and she would do so outside your uncle's will. You, as executor, would have nothing to do with the transfer to sole ownership. It was common for solicitors at one time to assume that a married couple would want property to be held in this way.
If, however, A & B own a property as tenants-in-common then whenever one dies the share of the property of the deceased will pass according to his or her will (or by intestacy) and need not necessarily go to the other joint owner(s). In this case you would have to determine under the will who has become the owner of your uncle's share in the property.
In addition, it should be noted that where property was owned under a joint tenancy it is possible at any time for one of the joint owners to sever the joint tenant unilaterally. So even if the property was originally acquired by your uncle and ex-wife as joint tenancy the joint tenancy could have been severed into a tenancy-in-common on, for example, his divorce and that this could have been done by either party simply declaring the tenancy was severed and giving notice to the other party.
The reason why the ownership needs to be clarified should now be obvious. If there was a joint tenancy then the ex-wife might be the sole owner and your executorship would have nothing to do with her rights over the property. So this needs to be clarified first.
You also mention
I need to get access to the property
but fail to explain why. Is this because the house contents form part of your uncle's estate and in whole or in part are left to others? Without understanding the reason for this comment it is difficult to comment further.
Please do not go changing locks on the property until you are sure whether the share of the property previously held by your uncle is properly part of his estate.
Eb