help advice needed about seeking professional help for olive tree

I have a 60 year old olive tree that i bought just over 2.5 yrs ago. I noticed after the first wiinter the leaves had started turning yellow,. We thought the roots may have got waterlogged as we had a particularly bad rain y season that year. The following yr i noticed that the leaves turning yellow had brown spots and identified it as leaf spot and remedies suggested using a fungus spray . I did this with roses fungal spray for 2 seasons but this year the tree has lost as many leaves as other years. I thinned the middle out last season as it was suggested that could be a reason for the problem. I did the tree at least 3 times this year in spring to summer after the recommended interval times, but it states not to do more than 3 in a year. It still has lost a lot of leaves and i dont want it to happen again next year. So where is the best place for me to seek professional help. I am, in Kent UK.

 

I'm guessing this was one of those very expensive knarled ones you see in large pots.
If so, did it come with a guarantee of some kind?

I often wonder how those plants fair long term.