OK let's take a closer look at the difficulties with this one :
1. It makes the brother an accomplice after the fact by silence about an extremely material fact.
2. It practically requires the murder to be pre-meditated at the time GR leaves Sheffield for his return journey as he would have not only to change cars but either leave his mobile behind or else take out the battery - otherwise he will have been picked up by antennæ on his return journey.
3. It requires a car to be available and enough cash to pay for fuel as he needs to do a round trip Sheffield-Bristol-Sheffield in the second car.
4. Anyone who knows the brother will know of the small number of potential cars that might have been used and the theory can be checked.
5. It requires the body to have been transported long distances in the boots of two different cars before dumping - one of which will have no good to contain any of Joanna's DNA.
6. It makes the decision to leave Joanna's personal effects in the flat very hard to explain. If Greg states that he found the flat with no sign of Joanna or of her keys, mobile, coat, shoes, etc, this will widen the field of potential supects considerably. (Potential answer is that the struggle left some ineffaceable signs in the flat, but even so...)
7. Greg would have known that he might not find Joanna at home - if she had had a reply to her text to the old acquaintance he wouldn't have - so he couldn't make detailed plans in advance.
8. I for one find it hard to see Greg planning to murder Joanna. If he did it, it needs to be quite unpremeditated...
9. It requires the body to be transferred between two car boots while probably in rigor mortis.
Do I dent your confidence yet ?
I didn't express any confidence, it was just a thought. Dealing with your points above:
1. Agreed, but how many unsolved cases are there where a relative isn't at least passively involved?
2. I don't agree. He may have just needed to sort things out that night. He might have just accidently left his mobile in the car he drove to Sheffield in.
3. Agreed, but his brother's car might well have had a full tank.
4. The police need to consider the possibility- have they checked?
5. I totally disagree- he drives back to Bristol in another car, arrives midnight, finds a situation he can't take, loses his temper, disposes of body that night then proceeds back to Sheffield. All DNA etc in the unknown car, which is the only one involved in the disposal.
6. I don't think it was a decision- in this scenario it is an unpremeditated crime of passion. He went too far in a rage, panicked, then decided he'd better get rid of the body, but none of it thought through, so stuff was left in the flat.
7. He just expected to find her at the flat- maybe she wasn't there when he arrived, which really would have sparked things off when she did get home?
8. I don't say this was planned, it just happened.
9. Again, see 5.