For Instance I have found this video by Kris Nelson of a class 800 passing through Slough station:
https://www.youtube.com/watch?v=zyeRVyP_2tc
Or this video by Leo Martin with a Class 800 departing Southall:
https://www.youtube.com/watch?v=zw5bJV4g4yk
Yep, the bottom one is mine. Caught a while back now on a standard move from North Pole to Old Dalby. The above move was the initial move from North Pole IEP Depot to Stoke Gifford, where that train remains to this day. To clarify, NONE of these videos are examples of IEP testing, they are examples of IEP moves and reolcations.
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Just a quick update, it seems that the GWR Class 800, reported as 800002 that visited Stoke Gifford for comissioning work, has returned to Old Dalby for Mainline testing. Maybe they will send another IEP down to Stoke Gifford, maybe not, or maybe one back down to North Pole again, it's all just guess work as it's kept 'above my paygrade'.
Anyone wanting to capture it, you would have to be pretty desperate! Here is the RTT routing it takes from Bristol to Old Dalby:
http://www.realtimetrains.co.uk/train/K97219/2016/06/10/advanced
It seems to head down the Great Western Mainline to Swindon, and follow all the way to Acton Mainline and Acton Wells Junction (usually using the fast lines between Reading and Acton as the slows are closed for Crossrail work during nights) where it heads on the North London Line up to Copenhagen Junction for the ECML, and then heads up to Peterborough on the slow lines, as by this point it's 7am and is dealing with Friday morning rush hour traffic on the ECML and at Peterborough heads to Old Dalby via its entrance at Melton Mowbray for around 10:39am.
Will do this in reverse if another is sent down to Stoke Gifford for further comissioning work (Which I see as likely, many visited North Pole for comissioning work).