My recollection is that Chiltern's franchise agreement includes an undertaking to obtain additional rolling stock as passenger numbers rise.
It's unusual for there to be competition for the same trains, but if the 170s are reaching the end of their lease with TPE, the owning ROSCO is acting perfectly reasonably if it favours a new long lease with Chiltern over a short extension with TPE.
Almost all the 170s are off-lease very soon due to the state of the franchising system, only the XC ones are secured for the next few years due to it being a more recent franchise award. The Anglia ones are off-lease from July this year, the LM ones are off-lease from September next year, the Scotrail ones are off-lease from November this year, while the Southern 171s go off-lease next July. It almost seems like Chiltern were spoilt for choice and opted for the ones that had a lease expiring a few months before they required them.
There has been competition for trains before:
* FGW secured some ex- CT and TPE 158s on short term loans and hoped to keep them in addition to their ex- Wessex 158s but Northern also wanted them and looked to have won the battle until they lost 158s to Scotrail.
* FGW apparently wanted to keep the 142s that they subleased from Northern but Northern needed more capacity and once FGW had additional 150s they couldn't put a good business case together for keeping them.
* Northern wanted to keep the 156s they lost to EMT but as a condition of getting additional class 150 carriages DfT made them release 4 x 156s.
Though the DfT can probably see the disadvantages to TPE of losing the 170s it's a privatised railway, and they aren't likely to tell Chiltern they cannot have the trains they want when they need them to meet franchise obligations, and they can't tell a rolling stock company that they've got to extend the existing lease with TPE. Perhaps TPE should look for some loco-hauled stock.
The issue TPE have is the franchise currently ends in 2015 and is set to be extended to 2016. Considering it'll only be for 8 months they won't want to fund a crew training program for a new type of train.
Stephen Hammond seemed to suggest that TPE will be able to hang on to 170/3s until the May 2015 timetable change (opposed to giving them up on 1st April 2015) and talks about hopefully Chiltern subleasing a few back until the end of the franchise in 2016.
There's also talk about Northern subleasing 158s to TPE and I can't see that working as no 158s will be freed up by electrification and TPE won't want a few 170s alongside a few 158s, while Northern can't afford to lose 8 or 9 trains.
So I think the best compromise would be Chiltern to only take 4 x 170s in May 2015 and for Northern to sublease 4 x 156s to Anglia who can then sublease their 4 x 2 car 170s to TPE. That way Chiltern gain capacity, Anglia gain a little (as 156s have higher capacity than 170s), TPE keep the same amount of stock and while Northern lose it would hopefully be a net gain if they have a number of 319s to use by May 2015.