As this article points out, London is likely to become more left wing after the local elections and recent headline cases have shown clashes between developers and left wing authorities which have stalled some major development schemes.  Whilst having every sympathy with the need to provide more affordable homes, authorities need to bear the following in mind:

• regeneration is expensive and with limited public sector resources developers need to find schemes attractive.  That means profit.
• the public sector simply isn't equipped yet to take on board and deliver massive development schemes across London. Private sector resources and skills are a necessity.
• the risk is the stalling of development schemes for years.  That can only add to the housing crisis.

Therefore, yes authorities need to drive a hard bargain to come to deals with developers that deliver on affordable housing targets.  But also bear in mind that development is good for jobs, for the economy and is the only way that additional housing is going to be delivered.