Here we go again, like this hasn't been discussed before....
First of all, how is it "serious"?
Secondly, they are only league tables. They've only existed for fifteen years, higher education managed without them.
Thirdly, they are completely different league tables and look at completely different things. It's therefore not suprising that there's a difference. The THES tables are biased towards the larger universities (eg. Manchester), those that are very research intensive, particularly in the sciences (again, like Manchester). This isn't really the case with the domestic tables, at least not to such an extent. Likewise, the domestic league tables often include things like student satisfaction and entry standards which, as far as I'm aware, the THES rankings don't. It's therefore not suprising that they differ so much.
The domestic rankings use the old (2001) RAE score. This is very out of date and even more so with Manchester (as it predates the creation of the "University of Manchester" as we know it post-2004). Before then it was the UMIST and Victoria University of Manchester, two seperate institutions. Since they've united it has created a new (and very large) research powerhouse. See the new RAE results, published just the other day. This will not be reflected in the new domestic tables next year. Manchester will probably climb a few places then.
Finally, just because a university is ranked 27th (or whatever) in a league table it doesn't mean there's a significant difference between number 12 and number 30 (something I've just been discussing in another thread).
This only goes to show that league tables are nothing more than a collection of what can, at times, seem like a random collection of statistics and factors, weighted in various different ways.