Here's what the first line says: as a political party called the Tories with regained confidence is slowly rising in popularity in the polls, blah blah blah. to inch means to move very slowly in a particular direction. This is evocative of an image of a worm slowly, inch by inch, crawling somewhere. So, inch ahead simply means move slowly in the forward direction. More idiomatically, it would mean something like slowly making one's way through something.
No. 10 is short for 10 Downing Street. That's the location of the headquarters of the British government in London. 10 is presumably the number of the building where the headquarters are found. You can read the linked Wikipedia article I provided if you really want to know what exactly it means.
In short, what this passage is saying is that the British public or the pollsters are afraid that come the next elections they will end up with somebody holding Marxist beliefs (socialist beliefs, in other words) in the government or something to that effect.