I am not a big fan of Dorries at all. She comes across as self-serving, even by MP standards, attention-seeking, a wannabee celeb.
She, to put it charitably, dissumulates - a lot. During her first brush with IPSA, she was forced to admit that her blog was "70% fiction", because her whereabouts in the blog conflicted with her expenses claims. As a consequence, it is difficult to know what to believe.
As far as the flat goes - If an MP lives some distance from the HoC, I am perfectly happy for them to have decent and reasonable living arrangements - a second flat/ house makes sense if you are working in a government role, or what have you. But if you only use the flat very occasionally, as Dorries herself appears to admit, why should the taxpayer be funding the monthly rental? Why does she not just get a hotel room for the night or nights in question?
One cannot help but think that the purpose of the 2nd home might be to feather her own nest - sub-letting at the taxpayers expense, so that definitely needs investigation.