Didn't notice you were in the UK. My other hobby is RC airplanes, and I salvage all kinds of junk to scratch-build my own planes. If you have an old broken umbrella (because it rains there all the time, right???), you'll often find that fiberglass rods are used for the ribs. They make great pushrods or struts for planes, but it might also work for your impeller shaft. Be careful cutting fiberglass rod - it will splinter if you try to use scissors or side-cutters. Use a Dremel with a cutoff wheel instead... and wear eye protection.
The impeller itself might be wallowed out, where the shaft goes through. This is often how the shaft gets broken to begin with, is by a worn out impeller that rattles around. You might cut a piece of inkpen tube to make a bushing for the impeller if it is worn out. Temporary fix, but it should get you through the night.