Loss of capacity of trustee

I have been approached by a family whose parents set up lifetime trusts. Against the advice of the solicitor (I’ve seen the written advice she gave) they appointed themselves as the sole trustees.

Dad died 3 years ago and Mum is now in a care home.

There are no helpful express powers within the settlement deed so I presume we are relying on statute now.

The beneficiaries are all discretionary (no IIPs) and include the settlor’s children and remoter issue.

Could we use s20 TALATA for the beneficiaries to appoint a trustee here or are we snookered by the fact that there may well be “remoter issue” alive but under 18? Is an application to the Court the only option here?