The injector wont be new unless the previous owner found a miracle stash of new ones...they've been NLA for many, many years. You can buy replacement o-ring seals (PM me) which perish but generally speaking the injectors last pretty well unless the car has been unused for a while.
The fuel pressure is managed by the built-in fuel pressure regulator which is in the hydraulic body, that's the same part of the assembly that the injector is fixed too. It's the square brass thing with 4 bolts, that is just south of the injector. This can't be replaced but for the same reason injectors fail (lack of use) they can break. If they do you have to find a 2nd hand replacement hydraulic body off a working SPi.
It is possible that your woes are because of fuel pressure, anything is possible but I've never seen it cause rich running before, the ECU is very good at managing fuelling and there are aftermarket kits for fuel pressure adjustment, even with those the ECU wont run rich. That suggests to me that something is telling the ECU that it needs more fuel.
Regarding the Lambda, is looks like you've proven that it is working. The problem with running rich for a long time is that the Lambda can get damaged.
Some basic questions for you, problem solving SPi's is about details!
1) You've not stated what your rpm is for cold to hot running. What happens? Is the idle steady in both situations? Is it hunting? This info may give some important clues.
2) You say you've checked and changed the 4x vac pipes (and fuel trap?) but have you checked all the other vacuum related pipes?? As you look at the SPi throttle body you will see 3x rubber pipes off the TB. There is also the brake servo pipe and the purge valve pipes of the rail. All of these can deteriorate and cause issues which will affect the fuelling of the SPi.
3) Are you absolutely sure the car is getting 88deg + ? If the car never reaches this magic number the ECU will continue to throw too much fuel in. Just because the temp is rising on the diag tool, doesn't mean the ECU will receive the message that it's reached 88deg. The earlier comment about flushing the coolant out and/or replacing the temp sender and/or thermostat is very valid. The temp sender that sits on the underside of the inlet manifold is hugely important to a good running SPi. The inlet manifold gets sludged up really easily and that messes things up. The ECU trusts the temp sender completely, so if the sender lies, its a problem.
4) You mentioned replacing the air filter temp sensor. Did you reset the error codes after you did this? You must, until you do so the ECU will run in limp mode (and over fuel), it will not auto diagnose everything is ok, it needs to be told. Any sensor error will put your SPi in limp mode (over fuel). You need to have an error-free system before you are working from a datum.
5) Have you set up your accelerator cable position / lost motion gap? If not, you should. During this process the stepper motor position is initialised. Look in the Haynes manual or the Rover workshop manual (which is on this forum) for how to do this. An old stretched/corroded accelerator cable can cause idling and fuelling issues.
6) what year is your SPi? there were changes made over the years, ones made in 91/92 are different to late 92 onwards.
I can probably think of more questions but that'll do you for the minute :)
If your SPi hasn't been well maintained then there is very high chance that your issue an accumulation of a few things, some small, some big.
