What is the difference between a 73mm and say an 83mm or 68mm ?

Is it just the plastic tube that goes down the middle between the cups ?

Reason for asking – CR have the ceramic 83mm at only £67.

ping

The size quoted needs to match the width of the BB shell on the frame that you are installing it into. Most shells are 68mm. 83mm is mahoosive, so probably rarely bought and therefore been gathering dust on CRCs shelves and they want to get rid of it. Measure your frame before you buy

Dunno, it could be convertible

http://www.chainreactioncycles.com/Models.aspx?ModelID=8243

By the shell do you mean the plastic tube down the middle ?

So that was my thought, CR have the ceramic BB in 83mm for £67 and a replacement tube for £9.99 so all in cheaper than buying a 73mm one

No – the shell is the metal bit on the bottom of your frame. Measure your frame (the bit where the BB goes in, from one side to the other) and get a BB to match