Hmmm! I have had to replace the black ties with a red one in the centre in order to take the chain off and give enough space for the tensioner to slide back in. It is all a bit tense at this stage, not only me but the chain is as tight as a bowstring and it takes considerable leverage to get the cog back on the end of the cam and the bolts done up, curiously reassembly has NOT proved to be the exact opposite of disassembly as Mr Haynes suggested. Nevertheless, it ran OK afterwards (silently!) at zero cost!