Hey Fozzer. That type of treatment from repair company deserves more that a rant...guess my rant and your rant may be 2 different things..
You can try a product like Pratley Steel maybe. If must be a very small crack if it is only seeping through slowly. Empty tank and sand the area a bit. Then use Pratley Steel. It is in tube form with one tube being a resin and other part a epoxy i think, and once you add the 2 together it sits like magic. Soft and easy to work and will foirm smooth finish that you can paint. Small can of model paint will do. The ones you get at art and craft shop used for painting model plastic airplanes etc. Comes in 1000 colours
Good luck