Mountain Ash dead - what to re-plant?

stravinsky

Hello,
I live in England and we did have a bad frosty winter. However the summer has been ok and our Mountain Ash has been healthy for 15 years. Today I got home and sadly it had collapsed, the tree was snapped at the base of the trunk. This summer it has looked poorly, the leaves have been sparse and curled up at the ends. Is this Fireblight? We had a Eucalyptus tree that also died this year that was postioned about 15 feet away. Any connection?
Our soil is of a clay consistency and the plants in the garden love it. What can we replace the Montain Ash with. Will the soil be diseased, should we avoid Apple tree since I've heard Apple trees suffer from fireblight too?
Th tree will be near a boundary wall and 15 feet from house so the roots must not be invasive/deep. Hope you can help. Our garden does look bare without our lovely trees.
Many thanks!
Stravinsky