It is a big thing at the moment magnet fishing, looks quite good fun aswel, there's a video on YouTube where a guy pulls a stolen motorbike plus numourous other items, how do you know what type of magnet to order and how much are they? And I wonder how long it will be before the council or the waterways clock onto it and tell you that you need a magnet fishing license

Check out Amazon, they have a wide range of sizes of the two basic magnet types - single, and double faced. You’ll get a fair idea of which to choose, by reading through comments left by purchasers. The price range for practicable magnets, is from just a few pounds to well over a hundred. Mine, together with 60 feet of rope, cost £20. However, I reckon I could have got away with half the strength, at half the cost. Mine has a 330lb pull; which for me, is somewhat over the top, as I don’t want to be pulling up anything that heavy; or rupturing myself in the process! Another problem being, if you hit an object flat on, it is difficult to pull off again - they really are powerful things!
You may be right about the licence thingy - seemingly the waterways people initially welcomed the sport, as it helped keep the waterways clear of debris. However, as it has grown more popular, it has begun to attract the usual set of numpties, who just leave scrap metal lying about on the towpath, rather than taking it away with them.