Sad to say it might very well be the Laburnum - 5 feet is really too close, and any tree, even a smaller one like this, is best planted a minimum of 25 feet from any structure with footings, and for large trees, 45 feet away. On the other hand, if your soil is clay (or the subsoil is) then contraction and expansion of the soil happens anyway - although the presence of tree roots will aggravate it.

Many thanks for your reply but it is not what I wanted to hear! I do not know if the soil is clay, I have only a small garden and the soil is different in various places.

Would it help if I trimmed back the laburnum tree as they say the width of the actual tree in bloom at the top is the same as the root spread - is this correct for any tree?

To a degree that's accurate, but its hard to predict where a tree's major roots will go - they usually tend to aim for damper places initially, so underneath paving or a hard surface is a good place to be because it remains moist. And after 14 years, its likely the roots are already in position beneath the slab anyway - stooling the top will not mean those roots wither or disappear, but may mean they don't go further.
I'm really sorry to have to say all this, I do know it's not what you want to hear. In the end, its your decision - laburnams naturally reach about 25 feet with a similar spread - its possible to keep them smaller by pruning back after flowering yearly, but I'd imagine, after 14 years, its a good size aready.

Thanks for your second piece of advice. My tree is about 20 ft. tall. I have just read in a gardening book that a trench, about 5 ft. from the trunk, can be dug and the large roots severed. Would this help to save the tree? Sorry to trouble you further - I won't bother you with this problem again.

well you can try it - it's one way to find out just how many and how large the roots are going towards the concrete plinth. And it's not a bother answering you...

Many thanks for your patience with my queries. I will contact you again if I have any further gardening questions.