Liberal Democrat leader Nick Clegg says he's sorry to see the departure of Brent MP Sarah Teather, but does not agree with her reasons for leaving
See also Brent MP to quit over policies
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Drivers belonging to the RMT union are due to walkout at 9.30pm on Saturday in a dispute over a sacked colleague.
A new opinion poll gives the party an increased lead at the expense of the Greens.
The Rail Minister said it was not a matter of crowd control but passengers "trying to get home after a long day at work".