There are several types of MPV out there but all the NR ones come from Windhoff.
There are 4 on Hs1 numbered 97011 - 97014. They are Windhoff MPV Master units. Here is an example:
http://www.flickr.com/photos/29199290@N07/5627527277
There are 13 units that were acquired for the WCML upgrade works 98001- 007 &98009 - 98014. They are fitted with a mixture of piling or OHLE maintenance equipment. Of these units 003,004, 009 & 010 are active. The others are in store. They may come back into use with the various electrification schemes that have been announced. They are all Windhoff Master units. Here is an example:
http://www.flickr.com/photos/darlo2009/6597097255 or
http://www.flickr.com/photos/52408438@N02/6675441267
98008 is a Twin Cab Windhoff Master unit which used to be an OHLE MPV but has now been converted for use as a High Definition S&C inspection and Plain Line Pattern Recognition unit. Here it is:
http://www.flickr.com/photos/darlo2009/6692356735
There are 32 Network Rail Windhoff Master & Slave units in operation. They are 98901-32 (Master units) & 98951- 98982 (Slave units). They work in semi permanent pairings so 98901 will work with 98951 etc.
They are used for RHTT works, both water canon & sandite, weed killing on the track side and on embankments, de icing and winter treatment works. Modules on the flat bed section of the vehicles are changed according to duty. Here is an example:
http://www.flickr.com/photos/bolckow/7030129811/
They are based at various location around the country. York Holgate, Slateford, Rutherglen, Inverness, Wigan, Tonbridge, Eastleigh, Horsham, Effingham and some other places I have forgotten!
There are other MPV units like the Geismar TRAMM operated by Balfour Beatty and others. You can see it to the left of this picture:
http://www.flickr.com/photos/darlo2009/7450654112 or here
http://www.flickr.com/photos/37318278@N02/6238310562