peterfranks it's a horrible job but doable. As sometimewoodworker said, a picture would help.
I had a few leaks with pipes inside the cavity wall inside kitchen copboards after my house build plumbers finished.
Just take the cupboard doors and hinges completely off, get some good lighting, a fan (hot under the sink inside a cupboard), a tool to scrape out the grout and a big drill. You'll probably break the tiles ....be patient and saying 'f***' a lot and blaming the Mrs helps. ????