So, the full story.
We updated a site to the latest version (2.6.1 at that point) to add a new site to it via MSM. That all went find, we added the new site, e.t.c., only to find the infamous relationship field problem was causing an issue.
I couldn’t fix this as the problem was happening despite there been no new fieldtypes installed, no rel references anywhere in the code or DB I could find.
After trying repeatedly to fix this, I gave up and noticed a new patch was available so I downloaded that instead and tried to install it.
After doing the normal installation procudure (copy files over, copy old settings files back in e.t.c.) the installation wizard ran like you would expect, ran for about 1 minute, then just stopped. There is no-longer anything displaying in the grey box, just a black horizontal line in the middle of the box.
Going back to /system and trying to run the installation again just again leaves you at the same enemy grey box screen. It looks like it has got part way through the installation and stopped but its not giving me any information about where it is storing it.
Hope someone can help.
Thanks