Maybe @Flakey know you likes some wheeled action Need to trim my steerer tube, will one of these in an appropriate size do the trick Or do I need to bring out the hacksaw gimp
Built several mountain bikes myself and I've used a pipe cutter to do this, but one of this type http://www.toolstation.com/shop/Hand Tools/d10/Pipe Cutting & Bending/sd2996/Monument Pipe Cutter/p27996. I didn't use a hacksaw as never seem to get a straight cut.
The gimps due an outing Or as my mate has suggested , juyst put a 5 mm spacer on.....why didn't I think off that !
While your all here why does more chain slip, I've changed the cassette thingy on rear wheel as the old one just gave up and spun. Am I just to strong.
Did you change just the cassette? If so that was where you went wrong, as if the cassette is worn the chain will have stretched and the chain rings will also have worn, so you need to change all three at the same time. As your living in France I'm sure there's an EU grant for this sort of thing.
I'm with @Pickles , pipe cutter rather than pipe slice. Remember to keep tightening the knob up tho so it doesn't get loose and cut crooked.