Finding witnesses who actually saw the Manchester bomber is quite difficult, I have found. Searching for interviews with survivors didn't lead me to any witnesses claiming to see him. I also searched for witnesses seeing the accused, which led me to find out that he was apparently identified by dental ID and bank card, but this evidence, as far as I know, hasn't been released to the public. This search also led me to find out that one woman apparently noticed him seconds before he supposedly set himself off due to 'risen bits', which was probably the shrapnel which was supposedly used. I haven't thoroughly looked at it, but the red she probably saw was probably the beginning of the explosion or his blood, because no red was found at the place where his shrapnel was found, with the exception of his blood. The only eyewitness descriptions of the bomber beforehand I could find were mentioned on what is likely police radio in the BBC documentary The Night of the Bomb. 26 minutes and 42 seconds in, it is reported that two members of the public saw the person responsible. They say he had a black beard, a black baseball cap and a rucksack. However, the surveillance photo of him supposedly on the lift to the bridge which connects the foyer and the tram does not show him in a black baseball cap. It is some other colour. I find it hard to believe that two witnesses would make the same mistake, and that the police wouldn't report conflicting accounts if they actually gave different descriptions, so I believe that the Manchester bomber did in fact wear a black baseball cap. So the only witness descriptions of the culprit I could find don't match the photos of the accused at the time, unless he, for some odd reason, decided to take off his baseball cap and put on a black one before detonating himself.

There is an International Skeptics thread on the bombing. I wonder if they debunk any of the Manchester conspiracy theories (I only read five pages or so)? Snopes debunks one video which claims that what the announcer says is enough to prove it a hoax. RationalWiki doesn't have an entry on the bombing.