Norton, I'm pretty sure you know, but just to make sure, for 20mm-ish, choose "double bolts" spacers like this one (sorry, French website, but you will find them in the UK for sure)
http://www.bps-racing.com/elargisseu...-12mm-m3a.html
...or at least some with offset centre bore like these ones.
http://www.bps-racing.com/elargisseu...-20mm-m3a.html