the mountain ash will be suitable as 20ft tends to be a maximum height and it has a light canopy. where as the ash can become a very large tree in time 40ft+.

I know that Walnuts don't flower until they are quite mature.....I have 40 years in my head (?) So unless you are a young whipper-snapper and don't intend on ever moving house, I wouldn't bother!

The mountain ash or Rowan tree is very suitable. It was traditional in Scotland to plant one near the garden gate for luck.
A neighbour in our last house, a row of detached bungalows, had a walnut in her front garden and it looked fine. I had a look on the internet and found this link below. Approach with caution is maybe the order of the day.

This tree Rita was the only small tree recommended for urban gardens.
http://www.graftedwalnuts.co.uk/rita.ihtml
Rita
A super hardy strain with very heavy crops. Should make a good choice for urban gardens due to its relative compactness and suggestions that in practice this variety appears to be self-fertile.

BAND: A
ORIGIN: Carpathia
TREE HABIT AND VIGOUR: Small, fast grower
NUT TYPE: Elongate and thin shell
KERNEL TO NUT RATIO: 54%
LEAFING ONSET: 25th April
TYPE OF FLOWERING: Slight protogeny
CROPPING: Heavy
AGE AT FIRST BEARING: 4 years

That's very interesting.....

Friends here have a walnut in their courtyard that is now a fair size and yielding a heavy crop of nuts each year. It wasn't there when I first moved in with them twelve years ago, or if it was it was small enough for me not to notice it.

So I think 40 years is very pessimistic......

They have given me an offshoot for my garden. It's struggling a bit after four years because after I planted it it sadly got buried in builders' debris. But it's looking more hopeful now, about 5-6foot tall.

I obviously drempt up the 40 year thing lol

dont know hy i spoke of ash and not walnut. i am losing the plot.

We knew you meant walnuts just a typing error .

I thought you were going wal-nuts Sbg ;-D Mindyou, so was I thinking they took 40 years to fruit lol

Thank you for your interesting responses.

The trees have now been planted and look rather pathetic as they are so small but from your comments I am expecting great things from them both!