Probate. What happens if it's not done?

[MsFanackerPants]

My DPs dad died in April last year. DP's mum still hasn't begun probate. A will was definitely made (DPs dad was a solicitor) but we have no idea what it says and DPs mum just can't or won't do it, even though family members who are also solicitors have offered to do it.
Are there any legal or tax problems she might encounter as a result of it being left so long. They had a joint bank account and it still is in both names as are household bills etc.

Yes. If the estate was above the Inheritance tax threshold your mother will still has to make sure the tax gets paid and it is within a relativley short time when it has to be paid. The HMRC will come after the money in an aggressive manner.

If it all goes to her then there won't be any IHT to pay.

Joint accounts don't pass under the will. She becomes the sole acc here on his death. Same might apply for the house and other investments.

We're not sure if it all goes to her. The estate would be over the IHT threshold but not if split between the couple, if that makes sense. At some point her DH inherited around £150k and had told DP and his sister that they would both be given money for a deposit on a house, however we don't know if that is set up through his will or otherwise. DP and I aren't in a position to buy yet, but I am working on the basis we shouldn't assume anything about the deposit which was mentioned.