My Braun Multipractic Plus basic is approximately 18 years old, but is still in good working condition. The knife has got stuck on the spindle, and I cannot remove it, and cannot find any food processor repairers locally. Grateful for any advice.

You could try putting a little cooking oil on it, let it sit for a while, and then rock the blade from side to side while turning it to remove. The rocking from side to side motion works great for lots of things that are difficult to pull off. Though I've never tried it with this particular device, it's worth a shot.