Loganberry looking odd

I was given a loganberry cutting by a plot neighbour last year.
I stuck it in a very large pot and it has grown very well.  Lots of flowers on last year's stem, but the leaves at the end are not looking quite right.
Some are coloured dark red and others have the wrong shape and are mottled.
Leaves at the bottom and this year's growth look normal.
Any thoughts as to what this could be. Viral infection?

Have you been feeding it and, if so, with what? Just in case it's hungry too