To fund my self-employment I have a part-time job (26 hours a week)

I have an Employment Contract which I have not signed as yet.
Is the contract still valid re periods of notice?
There is also a 'Restrictive Covenant' to the contract that states:
"During the twelve months following the termination of Employment, however that may arise, the employee must not in any capacity either directly or indirectly:
provide services or otherwise deal with any client in respect of business competing with that transacted with that client by him/her as an employee of that Company at any time in the period of twelve months before his/her effective date of termination"
That seems to me to mean I cannot work for a competitor and as there are at least 20 competitors to this company in Oxford PLUS I myself (as self-employed) am a competitor and the employer knew that when I was taken on, then surely such a clause is unenforceable.
Given that then surely it would be wrong to sign the contract?
And, given that is anything in the contract enforceable?