You might try "shocking" it. The technique that I know .....and there are members here that I am sure that know of other ways to do this... Is to take a shovel and at about the distance from 3/4 from the trunk to the end of the bottom branches drive it straight down with no other movement, take it out, and drive it straight down again doing this repeatedly all away around the tree giving you a circle. If you meet resistance use a rubber mallet or a hammer to help you drive it in. Now is the time to do it and do not delay. You have nothing to loose. If successful you will have to do this every year prior to bloom time. I will bet my bottom dollar that at least Bamboo or Tug( both very experienced) know about and or have done this technique or another similar to shock a tree into blooming. If threatening or sweet talking it won't work then give it a botanical slap in the face!

I have never used root pruning to encourage blooming, since it is often fatal, here in the desert. I'm not sure how Laburnum might react, though, since it doesn't grow here, either.
First, I would double check how much sun it is getting: most blooming trees need at least 6 hours of sun--ish, in the UK--a day during the growing season to produce flower buds. Next, I would double check my pruning habits: any pruning after midsummer is likely to cut off flower buds.
Also, is the tree simply not producing flower buds, or are they dying before they open? Severe winds or late frosts could easily do in the buds before they bloom.

Why are you feeding with tomato food during the winter months? This isn't the time to be feeding anything.

Thank you for replying,I don`t fancy root pruning (think this is what you are talking about Loosesrife2. Did it to slow down a Rowen that was a bit too rambant but it killed it. We are now going to try slow release fertiliser and see what happens.

Well, I couldn't recommend feeding it unless it was actually showing signs of nutrient deficiency. I.e., slow, sparse growth, yellowish foliage, abnormally thin twigs, etc. If the growth of foliage looks normal, compared to the others in the neighborhood, feeding won't help, and may actually harm next spring's bloom. Remember what I said about excessive pruning and sun exposure, too. Those are more likely to affect blooming than lack of fertilizer, especially since Laburnums are members of the pea family, and produce their own nitrogen fertilizer.

Agree with Tugb. the tree does not need feeding at all!