Letwin is one of the few Tories that I like! I think he'll be fine actually. The dodgy seats for the Tories are all the seats were Labour still polls a strong third. That gives scope for further increase in the LD vote due to Labour squeeze. Places like Taunton and Bridgwater are possible LD gains. Similarly I think tactical voting will start working against Labour this election in some seats, as it did in Chesterfield in 2001. Watch Cambridge!! LD gain!!
I know Dorset West very well, and Letwin is in big trouble (again)... he's seen as cold and aloof... if the LibDems pick someone who's very well known locally they'd pick up the seat easy enough.
What happend in Chesterfield was just bizarre... nowadays it's a mostly middle class (but of the progressive yuppy sort) commuter seat and Tony Benn was a *very* bad fit for the seat. So when he retired logic suggested that some fresh faced Blairite would become the Labour candidate and win with ease. However, Benn had purged the Chesterfield CLP of his "enemies" and they selected some old loony leftist dinosaur from the '80's. Who was also from London...