Weelittledrummer, I'm not sure about the suitability for disabled access. The park is described as 11 acres of "rolling grassy slopes", not ideal terrain by the sound of it, but the site linked above in the original post has a contact-link to the organisers, who would advise.
I have an unconfirmed report that the Rosenberg Trio may be appearing at the "Quecumbar" in London, also in July, but neither the venues' nor the Rosenbergs' site can verify.
If they are, it would be a superb chance to see them in a more intimate and access-friendly environment.

C-flat.