Hi Femur,
I managed to repair a tank leak (and many other problems) on my Powermax 185 boiler, although was advised to not bother by many people !
To be fair it did involve a serious amount of hassle and grief, it would help if you could describe or post a photo of your exact problem so it's easier to see the issue you have. The problem I had was I inherited a Powermax 185 with a long term leak on a tank connector, that was scaled on big time, and it took about 5 hours to loosen the bolt, and then replace the T-Pipe.
We all the know the nationwide reluctance of engineers to work on these boilers, so you need a lot of persistence and patience if you want to sort problems out with them (obviously all gas related issues to be addressed by a corgi engineer only).
Cheers,