Birch does not like heavy soils much, and particularly not damp clay, unfortunately, preferring lighter, sandier soils, so that's probably why yours is struggling. That said, this particular cultivar is slower growing than the species, but expected height after 10 years should be about 5 metres.

beech is a relatively slower grower than many trees. I would'nt add anything other than perhaps a sprinkle of FB& B.

I stand corrected - I said birch, but I meant beech, and otherwise, everything I said is the same;-))