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Blimey this could have turned out to be very serious :o.....https://www.gov.uk/government/news/overspeed-at-fletton-junction

Most permanent way has some inbuilt redundancy in its strength but a 25mph turnout is fairly short in length and the stresses at double the design speed can place excessive forces on fixing components etc.

I guess it's possible that if the signal clears when the driver is trundling at around 25mph between the 2 TPWS points then he could then start to accelerate, pass the second point just under the trigger speed and continue accelerating. 

I'm not suggesting this did happen, just that it seems to be possible given the description and the diagram. TPWS appears to be based on trains that are travelling fast on the approach to a hazard and need slowing in the event of the driver having a lapse in concentration, whereas this one wasn't.

You are quite corect.  In this case the TPWS calculations would be based upon a calculated maximum approach speed againt the required permanent speed resritiction ignoring the effect of any signals.  I'm not going to speculate further on what or may not have happend but this has always been a known risk.  TPWS cannot, and was not designed to, prevent things like this ever happening.

We don't yet know what went wrong in this case ... but I'm reminded from two overspeed incidents in our own area just how much worse the outcome can be ... and these are just two that sprung to mind

http://www.railwaysarchive.co.uk/docsummary.php?docID=421

http://danger-ahead.railfan.net/accidents/salisbry.htm

The picture in the linked page is a candidate for the 'Annoying / amusing use of completely irrelevant stock photos to illustrate press articles' thread, although isn't press article as such - I never knew they had 3rd rail electrical supply in Peterborough! Shame on you RAIB!!

Just seen the diagram in the report and what struck me was the curious sequence of speed restrictions the driver will have passed through in the space of less than 2.5 miles:

50mph on approach to PL1 at Peterborough (presumably followed by a stop at the platform)
70mph after the first set of points and then being slowed back to 25mph for the final set of points prior to entering the 105mph up fast.

Not seeking to speculate or incite speculation on the actual causes of this incident, just a general observation that jumped straight out at me from the diagram.

Obviously I've no idea what actually happened, but I'm wondering about psychological factors. We're told the train was about 20 minutes late arriving at Peterborough and was then held in the station. Could it be that the driver's consciousness of this was a factor?