Just to clarify, if the results are accurate, the car started life as a diesel (in 2004), then petrol, then diesel again (makes me laugh just typing it).
Then it's 99.99999999% certain that it isn't accurate

Just to clarify, if the results are accurate, the car started life as a diesel (in 2004), then petrol, then diesel again (makes me laugh just typing it).
There have been hundreds of brand new crated Ford Zetec E engines sold with no engine numbers on them, so it's quite possible. However, if there is evidence of the number actually having been filed/ground off then the origins of that engine are very suspicious.
Would the seller have an obligation to point out that the car has had an engine/gearbox replacement. I would have thought this would be the case, as the odometer would not correspond to what the engine has actually done, making the odometer reading pointless with regards to engine wear/servicing.
replaced engine= unkown service history
Engines always have number stamped on them. If they don't simple fact is someone out there has scrubbed it off and chances are they will have done it for a shady reason.
Surprisingly, Trading Standards and Citizen's Advice cannot give an answer to that question. According to the V5 the engine should be the original engine put in the car. If the engine didn't have an engine number would it still be stated as having one in the V5? If the engine has been changed shouldn't that be updated on the V5? I don't know how these things work but it doesn't sound right to me.As I have already mentioned, not always.
Best of luck with the car shoot1st, dealing with these things is an utter pain in the rear end.
Did you point out the notch in the gearbox to the garage? Keep your eyes peeled for then just dabbing a bit of paint over it!