Mr Thant wrote:

It would, as you mention, be impossible to run any peak Thameslink
trains through the six through platforms at London Bridge and still
provide a full service to Charing Cross, so we can rule that out.
Therefore, with only the current Charing Cross service to carry, it's
entirely possible the viaduct won't actually be four tracked until
2015.

Or run the existing two track service partially on the new viaduct while
modifying or maintaining the existing sections prior to or during track
remodelling. No doubt it'll all become clearer nearer the time!

Paul