I took my girlfriend back to the UK for a month back in April. This was her first visit to the UK and the cold/rain did not seem to put her off, even in Scotland! So I will take her back at Christmas so she can experience real depressing weather :-)

Anyway, her visa does not expire till the 23rd October but I would like to get the ball rolling on the next one. Does anyone know if there is a problem with applying for another visa and scheduling/attending a interview before her current visa has expired?

I cannot see any problem, don't cut corners with the second application, put as much effort as you did with the first.