Best way to cancel a previously agreed extension

[Spacecadet]

However for various project-related reasons (which I wasn't aware of prior to accepting the extension) I'd really like to walk away and not renew at this time.

So discuss this with the client, it might be something that can be resolved to your benefit.
If they're not willing to see your expert point of view then tell them that as the conditions/requirements/project direction has changed then you will not be able to formally accept the extension.

[tim123]

Exercise you right of substitution. You are ir35 compliant, I assume.

Even if (s)he is, there no guarantee that it has been achieved by having an RoS. Making this assumptuion is silly

tim

[lukemg]

Get with the programme fellas - ask for more cash and make sure you are on 2 weeks or no zero notice period.
They say no - you walk, they say yes, you have extra cash and a get out clause.