Not been paid...

[Willapp]

I don't think you answered about contract clauses relating to code ownership... my inclination would be not to release code until payment but it depends what the contract says. Certainly, refusing to do further work until you've been paid for what was already done is not unreasonable.

This was my first thought as well. When it comes to getting paid the only leverage you have is if you can legally withhold the source code that contains your work, otherwise you've given them total power over you. As the other replies have stated, this situation isn't uncommon when it comes to having vague agreements and is a lesson we all generally learn once. If they are otherwise good clients and there is a potential for further business, I would try hard to find an amicable solution and communicate in a positive way as much as possible. Try to emphasize (in writing) that the issues they have uncovered are not a result of your work (easily provable by performing the same tests on code before your changes) but you are happy to work with them to resolve the problems, perhaps at a heavily discounted rate or even free of charge if you are confident that it will lead to you getting paid in full and possibly future work.

At the end of the day the worst outcome for you is you get paid nothing for what you've already done - trying to take them to court won't guarantee payment even if you win, and unless the contract is very clear (sounds like it isn't) you can't be sure you would win. Best outcome is you get paid what you are owed plus perhaps something for the additional work, and walk away with your reputation intact.

[BlasterBates]

Yes they have found issues which are mostly historic and not a consequence of my changes

Fix them and then charge. The code control system will show when the bugs were introduced, or at the very least point this out.

Keep sending bills and go legal when they don't pay. They're using the system so it is fit for purpose.

If the customer has to maintain the code then you need to hand it over.