Hi thanks for your question. My name is***** should be able to help with this.
There are two key points here.
Firstly, you told your employer what you were intending to do, they agreed, and you resigned as a result. Hence, the Courts would not allow the employer to go back on thir consent to you doing this, as otherwise, you might not have resigned. This is called equitable estoppe. It prevents people going back on their promises/assurances where it would be unjust to do so.
Secondly, the clause you refer to prevents you approaching others. It doesn't prevent them approaching you - nor could it. Employers usually include a no-dealing clause, to prevent you working with clients, and if that isn't there, then you're often able to continue working with the clients etc. as there is no contractual restriction on you doing that.
In conclusion, therefore, you would appear to have very good arguments to say that you are not in breach of contract at all.
Thank you and apologhies for the slight delay in replying to you. I can't provide the service, but there are many law firms out there that would be able to help you. Any high street firm of solicitors should be able to assist you.
Please do remember to rate the answer for me before leaving... and thank you again!