do i need to drain unvented water cylinder to fix water inlet pipe of toilet

[Madrab]

Correct, no need to drain down the cylinder if all you are doing is turning off the mains and fixing the cold water inlet to the toilet.

If you are at all worried then once the mains is turned off then open a hot water tap and relieve the pressure in the cylinder but it isn't really needed.
No need to open the cold water taps either, just catch any remaining water in the supply pipe to the toilet when disconnecting once the mains is off. .

 

[Pizza123]

Thanks for clarifying Madrab. I really appreciate your help and advice.
Wish me luck.