Just so your clear Jon, legally you are entitled to put right any defect or wrong that you have made, it does not matter that there was no written contract, or anything like that. You should have name, telephone numbers, texts, email and start writing down your conversations with dates etc. you can pursue them in small claims court and you can put in for loss of earnings for your time spent in court too. Also if it goes over 30 days from job, the late payment act kicks in with interest, plus costs to take them to court.
For future jobs,and I'm assuming you never met the client, just done the job and billed them, it may be wise to ask for 50% upfront, we mever have any issues with this request, and balance after. The lads are right otherwise cash at end where poss.
Good luck mate