Now that my long running saga of 087/088 trouble codes is close to being resolved, it might be useful to describe the whole story.
Beginning mid September of 2012, at the end of a 800 Km day, I got a flashing glow plug light (GPL) and check engine light (CEL) and limp mode, all of which cleared immediately after re-starting. Over the following 6 months, the problem recurred randomly. Sometimes every two weeks and sometimes every 2 minutes. It was impossible to trigger the fault at will, although it did become gradually more frequent over time.
With guidance from TDI 101 forum threads and the boundless generosity of 2micron, I methodically substituted fuel pressure sensor, fuel pressure solenoid and fuel metering valve. Contrary to all expectations, it was the metering valve at fault and the substitute part has fixed the issue completely.
So away I go the dealer, part in hand, only to be told "not a VW part number, not a VW part - go to Bosch" Is this the ultimate VW brush-off or what? Searching internet sites, I found the part available in Latvia (!) and Spain. The part (Bosch #0928 400 706) is $68 ex Spain, where it's apparently a part common to the Seat brand.
Given the reasonable cost of this part and the 5 minute (including washing hands) of the R&R, it's not a bad place to start if you find a similar problem. Here's a useful description from the Spanish source of how the metering valve works:
The fuel pressure control valve comprises a fuel-cooled solenoid valve.
The valve opening is varied by its solenoid coil being pulse width modulated at a frequency of 1 KHz.
When the pressure control valve is not activated, its internal spring maintains
a fuel pressure of about 100 Bar. When the valve is activated,
the force of the electromagnet aids the spring, reducing the opening of the valve
and so increasing fuel pressure. The fuel pressure control valve also acts as
a mechanical pressure damper, smoothing the high frequency pressure pulses
emanating from the radial piston pump when less than three pistons are activated.