It covers the mainlines every 2 weeks and the majority of the rest of the network every 13 weeks, branch lines receive rarer visits. It of course cant go anywhere HST's aren't cleared.
Note that the original question is not just about the HST based New Measurement Train, but about Network Rail's fleet of various measurement trains in general.
Getting back to the OPs' more general question, I don't think that the New Measurement Train covers the Chase line on it's spasmodic wanderings around the West Midlands, however other Network Rail test formations will do from time to time, thinking particularly of NRs' class 150 based track assessment unit.
There'll be a need for some of Network Rail's more role specific test trains to cover the route every now and again, such as the Structure Gauging Train. As has been previously mentioned, partially due to speed limitations these sorts of trains often run at night when traffic is lighter. Every few weeks or so there'll be one or other of the Network Rail loco-hauled test trains stabled in the reception sidings at Tyseley: I'm not sure what routes these trains are there to cover on any particular occasion but the chances are that one or other will make it up the Chase line at some point.
Some more information on Network Rails' fleet of test trains can be found here:
http://www.infrastructuremonitoring.co.uk/fleet
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When is it coming up the valleys next and does it ever go down to Looe as that would be a challenge!
The Looe branch will see visits from Network Rail's class 150 derived unit rather than the NMT, which as has been said sticks to the main lines on a (more or less) fortnightly cycle. No doubt some of the loco hauled test trains, such as the structure gauging train, make it down the Looe branch from time to time, too.
I would have thought that the (presumably Cardiff) valleys would have been more probable territory for the 950 unit as well, rather than the NMT?
