How about growing something else? There are other small yellow-flowered trees.

My top recommendation is a Kowhai, New Zealand's national tree. Once they are sufficiently established, (took ours 3 years, its about 1.5m now) they are absolutely covered in yellow flowers for a few weeks in the spring (earlier than laburnum), and won't grow as big as a laburnum, at least not in normal British conditions, though might be different in a damp mild bit of the SW.

The most common kowhai grown in Britain is Sophora microphylla, but I grow the main alternative Sophora tetraptera, as it is likely to be slower and the crown less spreading. A number of cultivars are available, including some which are a small low bush, so be careful what you are buying. They are a bit tender, so not for growing up a hill in Derbyshire or a frost-pocket. Also probably doesn't like it very chalky. Our S tetraptera defoliates in the winter (on the Chiltern plateau exposed to east winds), but in milder places it can be evergreen, and it is also lovely as a foliage plant, with its mimosa-like leaves. Only downside is that the seed-pods are not very attractive. And it is just as poisonous as laburnum.
http://en.wikipedia.org/wiki/Kowhai

Some other not-to-be-missed small flowering trees with yellow flowers include:

Witch hazel (Hamamelis spp) winter flowering, completely covered in yellow if you choose a floriferous cultivar, though orange and rusty-coloured cultivars also available

Variegated Cornelian Cherry (Cornus mas variegata) late winter flowering, not a cherry but a dogwood, but the red berries are cherry-like, and edible, though not as good as real cherries. Variegated is slower growing than the species. Can be multi-stemmed, but we are growing ours as a single stem.

Other splendid small flowering trees (not yellow) include

Japanese Dogwood (Cornus kousa, several varieties available, with white to pinkish flowers) not just flowers but very showy large berries in late summer which are seriously delicious to eat, such out the squashy flesh through a hole in the skin). Good autumn colour. I think this could be my favourite small tree.

Judas tree (Cercis siliquastrum) - very slow, and took mine 7 years to flower, but very happy on chalk this one. Can be grown multi-stemmed or standard.

Lacebark (Hoheria spp.) Completely covered in white scented flowers in mid-summer after most other shrubs/trees have flowered. H. sexstylosa is supposedly also a good-looker when not in flower. H. "Glory of Amlwch" is more of a green lump out of season, but very tolerant of chalk. Can be evergreen in mild winters. Other species, cultivars and forms are available.
http://www.malvaceae.info/Genera/Hoheria/gallery.html Also a little tender, so not for the coldest spots.

[Farges'] [Harlequin] Glory Tree or Glorybower (Clerodendrum trichotomum var. fargesiae) Very sought-after small flowering tree with very late star-shaped white flowers, very showy blue/purple seeds in autumn and good autumn colour. Often misspelled as clerodendron so look for that too.