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all sorted - it was the break light switch. (note to myself: get the multimeter out before going into panic mode about parts and stuff...)

Second one is a vacuum switch and yes, looks to be for cruise control. Probably a standard fit item regardless of spec.
I've checked the voltage on the connector to this vacuum switch and it looks like there is current coming through. Wonder if this vacuum switch might also affect the brake lights ...

I've been on the phone with the local VW dealer parts department and they have kindly confirmed the part numbers:
The break light switch is: 6k0945515A

Then it comes the misterious blue switch, whose part number it appears to be 1H0907343
http://www.ebay.co.uk/itm/Audi-Seat-Skoda-VW ...

Short story is this: glow plug warning light keeps flashing, brake lights don't work. Went to Euro Car Parts and bought a brake light switch for a tenner, wrong part it seems (all done in a hurry, as the wife uses the car for work)
The problem I have is that there seems to be two switches above the ...