Thanks Ken for your input, it is appreciated.
I will sleep on it and let you know how I deal with this. Part of the problem with my 2.5.8 version might be that with the 2.5.1 I had a problem with the "Essential" template which messed my database up... I had to go back to the last backup and could not access certain activities on my courses.
I think that I will probably ask the administrator of the server to use the last backup and I will then use the 2.6. and then the 2.7 upgrades as you had suggested.
Some of my problems were sorted with the upgrade to 2.5.8 however, some "latent ones" might also be hanging around in the background.
Yesterday, I updated my wife's moodle database from 2.5.1 directly to 2.7.3 without a hitch... it might just be the 2.5.8 version or the latent problems from the Essential upgrade hanging around...
No worries, for every problem there is a solution, not necessarily the cheapest or the fastest to get back up and running.
With regards from Switzerland,
Byron