I'm so sorry delay, but I could not login to the UK site to access this question again, until just now. I appreciate your patience, as it was a situation that was out of my control.
Chopping the body up is very messy and they risk getting blood etc. on themselves that could lead csi to them.
---This is absolutely true, unless they thoroughly cleaned the area, got rid of all the blood, etc., but that would take time.
Rolling the guy up in a carpet is a nice touch .But that could have some problems as fibres from the carpet would be in the van they used to move the body in the carpet.
---This again, is true, IF they used a van to transport the body. Carpet fibres could be found by CSI as evidence against them. However, another scenario: they could carry the body in the carpet to nearby woods and burn it, then bury the ashes.
On to your other option using hydrofluoric acid.
1) How much of it would they need to complete the job? Would they be able to get that much into the guys flat without anyone seeing them?
They would need a few pounds of acid to complete the job. It would not be a problem getting it into the guy's flat, as they would have it pails disguised as something else (a fake label put on it).
2) Would they use the bath to do this?
Yes, they would do it in the bathtub.
3) If they killed the guy while in the bath as seen in the pictures in small speedos, could they just pour the hydrofluoric acid into the bath with the water?
Yes, they would pour the acid into the bath with the water.
4) How long might it take hydrofluoric acid to do the job, could they somehow use lye?
It would take a few hours acid to do the job and yes, they could also use lye or hydrochloric acid.
5) What would happen to the speedos the guy has on if they use hydrofluoric acid, or if they used lye?
The speedos would be dissolved, as well.
6) Would they possibly remove the guy’s speedos before disposing of him?
It wouldn't be necessary, as the acid would dissolve them.
7) When csi came to investigate the crime what would they find after the thugs had completed their work?
At first glance, they would find nothing, but if they were to take apart the plumbing from the bathtub, if it was soon enough after the crime, they might find residue of the dissolved body.
I hope this helped!