Self-employed - payment of invoices being witheld

Hi all

I have started working for a Company on a self-employed basis. I do not have a contract, I have just been told my daily rate and I invoice them as and when jobs are finished.

I got my first few invoices paid and now there are two they are refusing to pay for the following reasons.

I did 9.5 days work at a property and sent my invoice in. There were two of us on this job and the customer has said to this Company that she is not happy with a small portion of the tiling that was done. This tiling was not done by me but all of the payment of my invoice is being witheld until, I go back and rectify someone elses mistake. Is this legal?

The second job I have done 3 days work on and unfortunately the job has stalled due to not being able to get a scaffold tower. This is not my fault and due to this delay I cannot go back to the job for a couple of weeks. The company have now said if I do not go back they will have to get someone to complete the job and will not pay me at all, is this not blackmail?

I would appreciate any advise.

Many thanks
Matt