I recently bought a 2006 smax 2.0tdci and the first day I had it it drove fine, left it over night on an incline with front facing up went to start it in the morning and it fired up then cut out and wouldn’t start again. 
called rac out they plugged it in code came up p00087 fuel pressure control valve, he told me it was a generic code and that the issue was most likely the in tank electric fuel pump/sender as no fuel was coming up. Since then I have replaced the electric pump and the fuel pressure regulator but the car still won’t fire up no fuel is coming up from the tank. My mechanic is now saying the code says about low pressure at the fuel pressure rail, ive order a new fuel rail sensor but need to know if there could be anything else causing the issue? 
all fuses have been checked, in tank pump replaced and regulator replaced. My mechanic doesn’t think it’s the actual high pressure pump.

Any help will be greatly appreciated