what size jubilee clip  would fit do you think 
 

Here's a pic of a BR600 I re-assembled after a valve let loose.  There isn't any clamp, (as I re-call and see), just friction.
 
If the boot fits snugly, it should be ok as it's squashed between the block and carb.  A small zip tie  would work too if needed as there's just literally slight pulses..
 
 
 
 

that's great no I had a zip tie on it as It kept slipping of the carb I have ordered one from my local Stihl dealer only £6.00 I have a BR600 also so I can just use that and its better... hahahaha that's great pal once again thanks for your help glad I found this forum 

Your welcome!!
 
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