I think I’ll go with that Powerfix set and learn how to sharpen them properly.
Always a good plan. For repairing a badly dinged edge, a couple of large diamond ‘stones’ from eBay would do, for keeping a good edge, then some offcuts of MDF board, about 6″x3″, with different grades of wet’n’dry Spraymounted on either side will work just fine, dead cheap and easily replaced when the paper gets shagged out*, start at 800/1000, then 2000/4000, then finish with 6000 if necessary.
*As the paper gets worn, it gets betterer for putting a polish on the edge.
This is the system used by spoon carvers, etc, for putting an edge on knives and axes, my carving axes will shave my arm, so the same system will do a perfectly good job with chisels, for the purposes you need.
Here you go, three stones, coarse, fine and X-fine, for nine quid:
http://www.ebay.co.uk/itm/Neilsen-Diamond-Course-Fine-Extra-Fine-Set-Sharpening-Stone-150mm-6-Whetsone-/301936294681?hash=item464cce3b19:g:5vIAAOSw1DtXGN6G