Right so this is the results of the VAG COM Scan:

VCDS Version: Release 805.4
Data version: 20090602

Thursday,06,August,2009,14:53:36:62635

Chassis Type: 1M – Seat Leon/Toledo
Scan: 01 02 03 08 15 17 19 22 35 36 37 46 56

VIN: VSSZZZ1M Mileage: 69220km/43011miles

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Address 01: Engine Labels: Redir Fail!
Part No: 038 906 012 JJ
Component: 1,9l R4 EDC 0000SG 5539
Coding: 00102
Shop #: WSC 06402

5 Faults Found:
18080 – Coolant Fan Control 1: Open or Short to Ground
P1672 – 35-10 – – – Intermittent
18027 – Glow Plug Relay (J52): Open or Short to Ground
P1619 – 35-10 – – – Intermittent
19461 – Relay for High-Output Preheating of Coolant (J360): Short to Plus
P3005 – 35-10 – – – Intermittent
17957 – Boost Pressure Regulation Valve (N75): Open or Short to Ground
P1549 – 35-10 – – – Intermittent
17946 – Fuel Shutoff Solenoid (N109): Open or Short to Ground
P1538 – 35-10 – – – Intermittent
Readiness: 1 1 0 0 0

End

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Some of these might be irrelivent but as you mentioned it goes into limp mode occasionaly you might be looking at code 17957 as this is the N75 valve (the boost controller) which could throw the turbo into overboost and in turn limp mode!

With the codes now cleared you probably wont see the ECU light come on until you hit limp mode again, you can see the symptoms of each individual fault code here : http://wiki.ross-tech.com/index.php/Main_Page
You put the fault code number into the search box and it gives you a list of what it could be and how to rectify! The description for 17957 is:
Possible Symptoms

* Irregular behaviour
* Loss of power
* Limp mode

Possible Causes

* Wiring and/or connections faulty (short circuit)
* N75 faulty

Possible Solutions

* Check wiring and connections
* Check / Replace N75

You can get this info for all the codes but it might not help you yet! We need to see what code comes back (if anything!)

If / When the ECU light comes on again you are welcome to pop down and il read it for you again or you can buy this cable:

http://cgi.ebay.co.uk/VAG-COM-KKL-409-1-AUDI-VW-SEAT-SKODA-USB-CABLE-OBD2-II_W0QQitemZ310158179568QQcmdZViewItemQQptZUK_Diagnostic_Tools_Equipment?hash=item4836de3cf0&_trksid=p3286.c0.m14

Download the software from http://www.ross-tech.com (will post the proper link later!)

You will be able to read and clear the fault codes using this! (the software is a free download FYI!)

AmC