a large poplar tree has fallen in the garden - on cuting up the trunk it has large caterpillar like creatures in the trunk with parts being soft and spongy - any ideas as to what this is - thanks

It sounds like some kind of boring insect--there are a number of species, and Poplars are very susceptible to them. If the tree was dead before it fell, they may also be furniture or powder-post beetles which specialize in eating dead wood. Occasionally, the latter will also attack the heartwood of old trees, if there is an unsealed branch stub they can enter through.

Your sausage size friend is the larvae of the Goat moth "Cossus Cossus" it has been living in your tree with his mates for up to five years munching through the cellolose content of your trees wood, if there are other large poplar or willow trees in the vicinity chances are that they are also infected " if you smell the insect frass or the hole in the stem it will smell like a goat, hence the name" many other insects will be attracted to the effected area by the larvaes by products, indeed some butterflies get intoxicated by it.
This is an un common insect so should not be unduly persicuted the adult moth can have a wingspan of up to 9.5cm.