There shouldn't really be much pressure under the rocker cover, unless you have major problems with valve stem seals and piston rings. Anyway, it'll breathe out via the tube that goes from the rocker cover to the air inlet. TD5 ECUs do deteriorate. I've had considerably more duff ECUs than flat tyres in my 13 years of TD5 ownership. They don't make 'em any more, but there are second hand ones occasionally on Ebay. My experience is that they can be quite fussy. Mine refuses to start with anything other than an NNN500020, for example, and the other part numbers don't work. Land Rover wiring looms of that vintage deteriorate too. The insulation goes brittle and crumbly. The conductor wires aren't nice soft bendy copper but are hard and brittle too. Add a bit of oil and salty road dirt into the mix and its a recipe for unreliability. Another problem can occur in the wires between the accelerator pedal (or 'driver demand', as the manual calls it). This usually shows up as the engine going into a tickover-level limp mode and the yellow engine light showing. I stumped up for a new wiring loom a few years ago - the whole thing, not just the injector loom - and I've had much better electronic reliability since. Compared to the old one, the new harness felt really soft and supple, so I hope the materials are fit for purpose.
Also, I've discovered that it's possible for the ECU to look OK on the Nanocom but be useless for starting the car. I have two ECUs now, and it's very handy to have them to swap over for diagnostic purposes.
Oh, and if you end up swapping ECUs, don't forget to sync the ECU and the immobiliser unit afterwards, otherwise it won't start. On the Nanocom it's under 'utilities' and it's called 'learn security code' or sometimes 'learn DDS code'.
not to say the loom wont need changing, very tempted to just change it anyway, but rather go through one part at a time first
