I nee an employment lawyer

I was made an offer for a role in April and signed an offer letter. I have been working for the company whilst awaiting my visa to be be processed and signed an official contract of employment on 13th June. Today the company informed me that they will not be keeping me in work, from immediate effect. The contract stated 1 month notice must be given. They are saying this is invalid as they did not process the contract yet.
The company employed me on a part time basis and have since been wanting me to take on other duties and work full time with no change in pay. I believe this is why they have sacked me.